DIY Turtle Shell From a Foam Mat!

When a friend of a friend asked if I could make a turtle shell pattern for their child, I knew it was going to be a lot of fun. It's a simple but impactful costume piece that can be used for a number of different cosplay characters. You've of course got the Teenage Mutant Ninja Turtles, Bowser, Bowsette, Koopas, Squirtle, and finally, just a regular, normal, happy little turtle.

In this tutorial I'm going to show you how to transform a couple of puzzle piece EVA foam mats into a stunning turtle shell that will get tons of comments from pretty much everyone who sees it!

The templates I use in this instructable are available from my website and they include 4 shell sizes as well as 5 plastron sizes -yep, that's the front part of the turtle shell.

• Pattern: You can find it on my website. When you print it, measure against the print guides to know the scale is correct. ** When printing, make sure scale is set to ACTUAL SIZE** Because I make my patterns to work on both A4 and U.S. letter paper, often Adobe Acrobat will try and shrink the pages a little bit.
• Silver Gel Pen or Paint Pen: Used for any markings on the foam. If you use a ballpoint pen or sharpie and then try to paint over it with a light colour, the pen ink will migrate through the paint and you will never be able to cover the lines!
• Scissors
• Ruler
• Very Sharp Knife: If it is not really sharp you will have a terrible time when you are cutting the foam. I use a utility knife for this project. Use a brand new blade.
• Cutting Surface: Somewhere to cut where you won’t be destroying anything.
• 24” Square EVA foam Interlocking Mat: Shell- size extra small and small will require 2 mats, medium will require 3, large requires 4. Plastron- one mat is enough for each size
• Glue: I used both contact cement and hot glue in the video, and I think this works great, however you can use either type of glue exclusively if you’d like. They both have plusses and minuses, - I will go over that in the instructions.
• Heat Gun or really hot Blowdryer: Used for heating and forming the foam.
• Gluing Surface: A surface that you don’t mind getting glue on. A silicone baking sheet is great because hot glue doesn’t stick to it.
• Paint: Artists acrylics work reasonable well, though they can crack over time. If you want a really durable, long lasting finish, I would suggest using a flexible paint, such as Plaid FX paints.
• Rubber Gloves: To wear while applying the contact cement or paint.
• 150 grit Sandpaper: Used for rounding off the edges of the foam pieces.
• Latex Caulk (optional): Used for adding texture to the shell plates
• Wire (optional): Used for texturing plates. The wire I used was 2.5mm diameter telephone wire.

• The fumes from contact cement are toxic, so use it in a well ventilated space and wear lung protection such as a respirator.
• Whenever you heat foam (with a heatgun or blowdryer) there is potential for the foam to release harmful gases, so use a respirator and do it somewhere with good ventilation.
• Some EVA foam contains a chemical called formamide. There are some people that say there isn't enough formamide in EVA mats to be harmful, and others that say there is. Do your research and come to your own conclusions. At the least, I would say it is a good idea to open your foam mat up and let it sit in the sun for a day or two, as most of the chemical will off-gas from the foam. Or buy foam that is labelled formamide free.
• Sharp knives and hot glue can cause injury. Be sure to use in accordance with the manufacturer's instructions.

Disclaimer: If you rely on the information portrayed in these instructions, you do so at your own risk and you assume the responsibility for the results. You hereby release Lost Wax Designs from any and all actions, claims, or demands that you, your heirs, distributees, guardians, next of kin, spouse or legal representatives now have, or may have in the future, for injury, death, property damage, or any other liability that may result related to the information provided in these instructions.

Before you start printing the PDF you are going to want to decide what size of shell you want to make. Currently there are 4 Sizes, form extra small to large. One thing to think about when deciding your size is whether you want to be able to sit down when wearing the shell. If so you will want a bit of a smaller size. The shell measurements for the different sizes are below:

• Size Extra Small: Height: 18" (46cm) Width 15" (38cm)
• Size Small: Height: 21 1//2" (55cm) Width 17 1/2" (45cm)
• Size Medium: Height: 24 1/2" (63cm) Width 20 1/2" (52cm)
• Size Large: Height: 28" (71cm) Width 24 1/2" (62cm)

Print the pattern pages for the size you want to make from the PDF. Make sure the scale is set to “actual size” or 100%. I make the patterns to work on both A4 and U.S. letter size paper, so that can confuse the computer and make it want to scale the pattern down. After printing, measure the print guides with a ruler to verify that they are the correct size. (If you need a program to print from, I find Adobe Acrobat Reader DC works well, and is free)

*Make sure the scale is set to actual size in the settings when you print.**

The turtle shell pattern is larger than one sheet of printer paper. Line up the “+” marks on the printouts and tape the sheets together. I find it is easier to line up the marks if I hold the papers up against a window so that the light coming through the window allows me to see through to the lower registration marks. If you have a glass table, you can put a light under it or you can even use a white computer screen.

***When you are taping, make sure you put tape across the actual pattern, otherwise when you cut it out, there will be nothing holding that pattern segment together!***

Use scissors to cut out the paper pattern, just barely leaving the black line visible.

There may be dotted lines on some pattern pieces, these are just if you wanted to make an alternative shape that is more contoured at the top.

Lay pieces 8 and 9 down on one of the foam mats. You will be tracing them once right side up and then flipping them and tracing them again. Make sure you have them arranged with that in mind.

Use a fine paint pen or gel pen to trace the pattern pieces onto the foam. I use a paint pen because if you use ball-point or sharpie pens, they will bleed through any light colours of paint and look ugly.

When you remove the pattern, extend the alignment marks to the inside of the traced pattern, and write the letters beside them for reference. There are also some non-lettered marks which you should also extend to the inside, these are just for spacing the outside rim pieces at the end.

Once piece 8 and 9 have been traced right side up, go over the alignment marks with a ball point pen while the paper pattern is on top of some foam. This will allow the pen to indent the paper where you are marking. Then you can flip the paper over and draw over those indentations so that you can see where the alignment marks are supposed to go.

Trace the flipped versions of piece 8 and 9 onto the foam.

When writing the piece number onto the foam after tracing, I suggest adding an “A” to any pieces that were traced right side up, and a “B” to any that were traced flipped. It makes it much more obvious if you are about to make a mistake when gluing them together!

Using your sharp knife, cut out pieces 8a, 8b, 9a and 9b. Cut directly on the lines, and make sure the cuts are vertical. Don’t try to cut all the way through the foam in one cut, rather repeat the cut a couple of times with moderate pressure.

If the cuts look rough, that means your knife is too dull. Either get a new blade or sharpen your knife. It’ll make a ton of difference.

**Remember to watch where your fingers are at all times and make sure you don’t cut them!

I normally use hot glue for my projects, as I am a bit leery of breathing the fumes that go along with contact cement, however for this project I decided to use contact cement for making the Shell Base as it was much quicker. Make sure to test your contact cement to make sure it holds well to EVA foam. I have tried some that were actually quite weak. I have heard that Weldwood makes a decent contact cement that is available in the U.S.

When using contact cement, you apply glue to both sides of what you want to glue, then you let them dry separately until the glue is no longer tacky. I like to apply a second layer because the edge cuts on foam are quite porous and some of that first layer gets absorbed by the foam. When the second coat is no longer tacky, press the two parts together firmly. When the two parts come together, the glue instantly and permanently bonds. This makes it important to have your pieces lined up properly because you only get one chance! I also like to spread the contact cement with a bit of scrap foam off-cut or something similar. This helps to get a nice even layer of contact cement.

**Use contact cement in a well ventilated area and wear appropriate lung protection!**

You can also use hot glue for this. When using hot glue, the best way to get good results is to turn the temperature of your glue gun as low as it can go while still being able to squirt out hot glue. That’s the reason I always recommend having an adjustable temperature glue gun.

When using hot glue, glue small sections at a time, and then make sure to hold them together until the glue is completely cooled. The biggest mistake novices make is to let go of the seam too soon, so make sure you don’t make the same mistake.

Using whichever glue you prefer, glue the four shell base pieces together, gluing two at a time to make two halves, and then gluing the two halves together. As you glue, the shell base will start to make a concave dome. Take special care to make sure you have a good glue bond at the outside end of each seam, as those will have the most stress on them.

Once glued, the shell base will likely be inside out, so carefully invert it so that the smooth side of the foam is on the outside of the dome.

You can shape the shell base a little bit at this stage, with a heat gun to soften the foam (as long as you used contact cement. If you used hot glue, the seams will come apart with the heat)

Layout the plate pieces (pieces 1 to 7) on a second foam mat, but this time flip the mat over so that the textured side is facing up. Trace all the pieces, remembering to mark the centre lines on pieces 1 to 4, as well as the top of each piece for reference later.

Pieces 5-7 get flipped over and traced a second time.

When cutting the plates, angle your knife at about a 45 degree angle. This creates an inward leaning bevel once the plates are flipped over to be right side up.

Cutting at an angle on a textured surface will leave an unwanted wavy edge, so after all the pieces are cut out, you can flip them smooth side up and cut just the tiniest little strip from the outside edge all the way around.

As your markings are currently on the back of the plate, you will need to move them over to the front of the plate. Make sure the centre lines are long enough, as part of them will likely get sanded away in the next step.

After being cut out, the plates will have fairly hard, square edges on the bevels. If you want, you can leave them like this, as that does save a fair bit of time. I really like the effect of rounding over the top edge of all the bevels on the plates.

To round the edges, take some 150 grit sandpaper on a wood block and sand off the edge all the way around each plate. Sand in the same direction as the edge you are trying to round off.

After sanding, the edges are going to look a bit furry due to the nature of foam. If you use a heat gun on the foam, that will reduce the roughness of the sanded areas.

**Note: if you are using low density foam such as that found in some camping mats, sanding isn’t going to work and will just result in a mess

You can also spend a little extra time with the heat gun on the foam after you have smoothed it out, heating up just enough so you can form it. Use a rounded object such as your knee to form each plate over, so that it will conform better to the convex surface of the shell base.

I used hot glue to glue the plates to the base. It seems to be the easiest, cheapest, best way to do it. I used a bead of glue all around the outside edge of each plate and then a quick swirl in the centre and that seemed to work well. The main thing is that the outside edges are well glued.

Before you glue anything, do a dry run and lay out the plates on the base so you can see if there are going to be any problems. It is likely that, due to inconsistencies when cutting and shaping, the plates won’t all fit together absolutely perfectly, so you’ll need to average out the spacing so things seem reasonably even. As you glue, it’s possible that you might even want to re cut an edge or two. For sizes small and medium, I left a gap between the plates, however on size extra small, I pretty much glued the pieces edge to edge. Decide what works best for you.

Start with Plate 2 and glue the bottom edge directly on top of the horizontal centre line, lining up the centre marks on piece 2 with the vertical centre line on the shell base.

Glue piece 3 under piece 2

Glue piece 1 above piece 2 and piece 4 under piece 3.

Now glue pieces 5,6 and 7 down the side starting with 5 at the top and working numerically down. Repeat for the other side with 5b,6b, and 7b

This step is to make the rim of the base flare out.

Make a cut all the way around the base that follows the outside edge of the plates. This cut doesn’t need to be super deep, maybe 1/3 of the way through the foam. Cut a second line a millimetre or two below the first line but angle the cut so that you are ending up with a “v” shape cut. Remove the sliver of foam you make when cutting this line.

Squeeze hot glue into the V cut you just made, about a foot at a time, and push down on the shell as the glue cools. This pushes the bottom edge out and the glue holds it in that position. Repeat this all the way around the V cut until everything is glued.

Next it is time to add some small plates around the rim of the shell.

Measure the width of the shell rim between the outside edge of the plates that are glued on and the outside edge of the rim. The rim is likely going to be wider and narrower in different parts. Figure out what the narrowest measurement is and use that to determine the width of the strip you will cut next.

The strip widths I used were- Size XSmall: 25mm, Size Small: 30mm, Size Medium: 35mm

Note: I say narrowest part, but if there is just a small section that is a little narrower than the rest of the rim, you may not want to go quite that narrow, it just means there will be a little overlap in that spot

Using whatever foam you have left, cut strips that are as long as possible, and the width you just decided on. Just as you cut the plates from the back side of the foam, do the same thing with the strips. Cut them from the back and taper the cuts inward as well.

Sand the bevels on the edges of the strips if that’s the look you are going for.

There should be marks all around the rim from when you traced the pattern. Use these as a guide for where to place each rim plate.

Cut the strip into segments that match the marks around the rim, making these cuts also at an angle. I then like to make another cut to cut off the square corners on the outside of each rim plate. Sand these newly cut areas as well.

Glue each rim plate in place around the rim.

The very bottom rim plate will need to bend around the pointy end of the shell, so heat it up and shape it before gluing.

Once all the rim plates are glued, there will probably be some extra edge of the rim from the base sticking out. Cut the protruding base rim off so it is flush with the rim plates.

Lastly, cut notches out of the base rim to correspond with where the corners of the rim plates meet up.

You can build up a texture on top of your shell using some latex caulk and using tools to create the look you desire. This gives a great, organic look, but adds a bit of weight, as well as needing to dry overnight.

• Squeeze some caulk on one of the shell plates
• Spread the caulk over the plate with a scrap piece of foam. It's pretty much like icing a cake. You don't need to get it smooth, but you do want to make sure you have a fairly even thickness of caulk over the plate.
• Find an implement that is smooth but a fairly narrow. I used the back end of my scalpel handle, but other ideas would be cutlery handles, skewers, etc.
• Use that implement to etch concentric shapes into the caulking, pushing it aside so that you create some deeper paths.
• Wet a paintbrush a little and paint around those concentric shapes. This smooths the ridges and valleys together so that everything looks...nicer.
• The texture of the paintbrush will also be imparted to the caulking, so if you want a more agressive texture, use a brush with stiffer bristles.
• Repeat for all the plates.
• You may also want to apply caulking in the gaps between plates. A wetted paintbrush works well to smooth out that caulk in the cracks.
• Allow the caulk to dry at least overnight.

This texture adding method takes about the same amount of time as the previous one, but adds no weight. It gives a slightly cartoony look to the shell.

One of the great things about EVA foam, is that you can heat it up and then press a texture into it. If you reheat the same area, the foam will relax and the texture will disappear. Here's how to make a stamp to stamp in a texture.

• Start by tracing piece piece 2 on some thin cardboard.
• Cut the cardboard leaving an extra cm or so all the way around
• Cover the cardboard with double sided tape
• Using a piece of wire, start coiling it from the outside perimeter of the traced pattern piece sticking it down onto the double sided tape as you go. Work your way inward until you have a spiral of wire. This is your stamp. The wire I used was 2.5mm diameter. (telephone wire)
• Place some support under the shell so that you can apply pressure to the top of the shell without damaging it. I used a bag of scrap foam pieces. You could also use a bundle of blankets or towels.
• Heat the plate on the shell that you are going to texture with your heatgun.
• Press the stamp into the shell plate, putting much of your bodyweight into it.
• Hold the stamp in one position without moving it until the foam has cooled.
• When you remove the stamp, there should be an impression from the wire coil.
• If you are not happy with how the plate looks, you can reheat it and the texture should disappear.

You can use the same stamp to texture plate 2 and 3 on the shell. The thing to think about though, after you have one plate stamped is that you don't want to reheat that plate again or you will risk losing the texture you just added. Make a barrier that will block the heat from whatever plate you are trying to heat. At the beginning, you can just use the stamp itself, held vertically as a guard, however, as you go, you will need to make more complex guards from cardboard.

Once you have textured plates 2 and 3, trace piece 4 onto your stamp, then remove the wire from the tape, and re-coil it to fit inside the new shape, trimming off any extra. You can use this stamp for shell plates 1 and 4.

Continue to work your way from largest to smallest plates, tracing them onto the stamp and repositioning the wire as you go.

For the thin shell strips around the edges, instead of coiling the wire I just put three pieces of wire on as shown in the picture.

I happen to really like the turtle shell with a smooth finish, so for that shell all I did was add some caulk in between the shell plates to fill and smooth any gaps.

I used 3 different painting styles for the three different textures, so I am going to share them all with you, going from quickest to most time consuming.

Airbrushing is the quickest and looks great, but it's not something most people have, so I'm not going to cover that here, just thought I'd mention it.

To get a similar look to airbrushing, I used a dry brush painting technique as follows:

• Paint the shell with two coats of green acrylic paint to give it a good base coat and let it dry. I like Plaid FX paints because they stay flexible and paint on nice and smooth.
• Add black or blue to the green paint you used for the base coat to get a darker shade of green
• using a large brush, dip into the darker green paint, but only get a little on your dry brush.
• dab the brush on some scrap paper, cardboard or foam to remove any extra paint from the brush
• Dab the brush into the cracks and around the edges of each plate, slowly building up the darker paint in those places to give some depth.
• Once the darker paint is dry, use a yellow or yellowish green paint and use the same technique, this time starting in the centre of each plate and fading the yellow as it moves out from the centre.

This is how I painted the shell that I textured using the acrylic caulking.

• Give the shell 2 brown base coat layers and let dry
• Dry brush a darker brown into the cracks and a little bit up and around the sides of each plate. Let dry.
• Using a small paintbrush, paint either very dark brown or black into the concentric grooves on each plate.
• Before the black paint dries, wipe across the top surface to remove any black paint that is not in the groove. Let dry.
• Put on a glove. Put a little bit of a lighter colour of brown paint on a piece of cardboard.
• get just a little paint on your finger and then wipe most of it off onto the cardboard.
• Lightly rub your finger over the top of the plates, leaving paint on all the high points of the texture.

This is how I painted the shell that I stamped the spiral texture into.

• Start with a base layer of two coats of either black or dark green. Let dry.
• If you used black as a base coat, paint some dark green along the indented spirals.
• Before the green paint dries, rub across the top of the plate with a little square of EVA foam to smear/remove any of the green paint that is outside of the spiral indentations.
• Squeeze a little lighter green paint onto a scrap of EVA foam to use as a paint palette.
• Using your foam square, dip the edge into the paint, rubbing it along the foam palette to spread the paint on the square and to remove most of the paint.
• Rub the foam square onto the shell plate, depositing paint on the plate. You just want a minimal amount of paint on the foam pad so that it does not flow down into the spiral indentations.
• Repeat this process until you have built up paint on the plate. It doesn't need to have complete coverage, in fact you actually want it to be somewhat patchy as this gives more of a natural look. Let dry.
• Use a different color of green and do the same process over the previous coat. I used a slightly darker green than the previous coat. Let dry.
• Add some highlights with a yellow or yellow/green paint. Apply in the same way with the foam pad, but just do it on some of the edges and the centre of the plates.
• Lastly, add a dark green over top of everything in the same manner.

Yay! You are finished your shell. Here are a couple of pictures showing my finished shells. The picture shows 3 of the four shell sizes, extra small, small and medium. Large is not shown.

If you'd like to make the plastron as well, keep reading!

If you’d like to make the plastron for your turtle shell, I have added some templates to help you out. Making costumes that fit many different body types is a bit tricky, so for the plastron I have a wider and narrower version for each size. For the sizing, I am just going to include length and width measurements and you can decide which size will fit you the best. Just be aware that the size small Plastron doesn’t necessarily go with the size small shell. A couple things to think about when choosing you size are:

Do you want to be able to sit down in your costume? If so, make sure the plastron doesn’t extend much past your waist.

Do you want to be able to put your arms out straight in front of you? If so, put your arms out straight ahead and measure the space between them against your chest. This will be the maximum width you want the plastron to be. 

• Size Extra Small (Wide): Height- 13” (33.1cm) Width- 9 1/2” (24 cm)
• Size Extra Small (Narrow): Height- 13" (33cm) Width- 7 1/3” (18.7cm)
• Size Small (Wide): Height- 15 1/4” (38.9cm) Width- 11” (28.3cm)
• Size Small (Narrow): Height- 15 1/4” (38.9cm) Width- 8 2/3” (22cm)
• Size Medium (Wide): Height- 18” (45.9cm) Width- 13” (33.4cm)
• Size Medium (Narrow): Height- 18” (45.9cm) Width- 10 1/4” (26 cm)
• Size Large (Wide): Height- 21 1/3” (54.2cm) Width- 15 1/2” (39.4cm)
• Size Large (Narrow): Height- 13” (33.1cm) Width- 12” (30.6cm)
• Size Extra Large (Wide): Height- 25 1/4” (64cm) Width- 18 1/4” (46.5 cm)
• Size Extra Large (Narrow): Height- 25 1/4” (64cm) Width- 14 1/4” (36.2cm)

I have also positioned the parts on the template so that they are all oriented vertically. This way, if you find you want to alter the length to width ratio, you can scale the pages on just one axis. 

The assembly is very similar to the construction of the shell so I won’t go into a ton of detail. Here are the steps.

• Print, making sure to print at actual size
• Cut out the pattern
• Trace the pattern pieces onto your foam mat, flipping each over to trace a second, mirrored version
• Cut the pieces out of the foam, cutting vertically (not at an angle like you did for the shell pieces)
• Sand around the edges of all the pieces to give a nice rounded look.
• Heat the edges to smooth them, and form each piece over your knee to give it a slight dome.

• Glue the pieces together in sets of two to start- ie glue piece 1 to it’s mirrored partner etc
• Glue all the sets of two pieces together to make the plastron
• There will be a small diamond shaped gap in the centre between the 1 and 2 piece sets. Cut a small piece of 2mm craft foam and glue it over that hole from behind.

This is the same process I used to dry brush the turtle shell. I started with a yellowish orange base coat and then added a slightly darker colour in the crevices.

This part can seem the most daunting, and I spent a lot of time thinking about the best way to go about it. What I finally decided was the best was to use an actual backpack that gets glued inside the shell

Old backpacks are pretty easy to come across at thrift stores for a couple of bucks, and you may even have one in a closet somewhere at home. The best pack is one that will fit completely inside the shell without sticking out the top or bottom, and if you can find a green or brown one, even better! What I like about using a pack vs. gluing some sort of strap contraption right to the shell, is that, with a backpack, you have a large surface area that you can glue down. That way, the stress on the straps gets spread through all the glued surface rather than being concentrated on one small strap attachment. Also, the turtle shell is a concave shape, and (hopefully) your back is not! The bulk of the backpack can fill that gap a bit and this makes it much more comfortable to wear. 

One thing to think about when gluing the pack inside the shell, is that when worn, the pack will likely slide down a bit, so don’t glue it right at the top of the shell if possible. Glue it a bit further down.

I used hot glue to glue the pack to the shell and that worked well. One benefit is that if you need to reposition the pack in the shell (I did) you can heat the pack up with a heat gun and move it.

I also cut off extra pockets and straps that just added to the build and would make it harder to paint the backpack afterwards.

Once the pack is glued to the shell, if you'd like, you can paint it green or brown so that it fits in with the costume colours.

The easiest solution for attaching the plastron was to glue a strip of velcro to the front of each backpack strap. I then glued strips of velcro down the edges of the plastron. This way, once your pack is on, you just stick the plastron to the straps and you are good to go. Quick and easy. This works better for a child as the ratio of backpack straps seems to work out better. 

As you get into larger sizes, the backpack straps don’t extend down as far before you get to the plastic webbing adjustment dongle things. For larger sizes, it works best with a backpack that also has a waist belt. You can attach a small piece of velcro on each strap where it would correspond with the top corners of the Plastron, then attach a piece of velcro in the centre of the waist strap on the backpack as well as glue one in the centre of the plastron where it would come into contact with that piece. So the two top velcro strips hold the top of the plastron and the third on the belt secures the bottom. Another option would be to do a bit of sewing and change that webbing adjusters so that they are located near the bottom of the webbing instead of right where the webbing meets the thicker straps. 

If possible, it’s great if you can sew the velcro to the straps, as this will give much more strength than just using hot glue.

You did it!! Now you can enjoy an awesome costume you made yourself:)

Just for a bit of size reference, In the picture, my wife is wearing the shell size medium and plastron size large, and my daughter is wearing shell size small and plastron size medium.

Thanks for hanging out with me!

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