How to Build a Fiberglass Sub Box

In this Instructable i will show you how to build your very own custom subwoofer box!

Depending on your install you might have to remove some stuff from your trunk or cabin (spare tire,seats etc...)

In my case i have a single cab truck so my options are limited. I decided to take the bench seat out and install some bucket seats.

So i used the old bench seat mount and welded new bars across them, the new seats will mount to these and so will the fiberglass box.

For this step we are going to cut a rough shape following the lines of the seats, Using cardboard for a template trace your new design onto the MDF

Now we can cut a front, back and bottom.

Looking good so far...

Now we can trace the outline of the seat onto the box.

I wanted it to look flush with the seats,also i didn't want to be hitting my elbows while driving so i cut as close as possible to the seats.

Then we use a router and rabbet cut all the way around the top edge off the box. This is necessary so when wrapped with fleece you will have some where to staple and will result in a lot less body filler.

We need something mount speakers too!

So i made a simple circle jig out of plexiglass and attached it to the router.

Now we can cut four rings for the subwoofers.

Make sure to cut the outside of the ring first but don't cut all the way around the ring, leave 1 inch tabs on both sides of the circle (these will be cut with a jig saw after you cut the inside ring out)

Next i bought some hardboard (hard bendable cardboard) from a upholstery shop.

For your speakers to mount flush you need to put your speakers face down and put the ring on top of the mounting holes.

Then cut strips of the hardboard to the height of the speaker edge and the ring, While your rings are still around the speakers staple the strips to the ring making sure not to staple into your speakers!

Now we mix up some resin and cut some fiberglass cloth into strips. First apply resin to the hardboard and then lay the strips of fiberglass and finish coating the cloth. I decided to resin the rings also but this is not necessary.

We need to mount the rings and depending on how your box is shaped this step will be slightly different.

For my install i cut small strips and cut an angle on the the end so i could adjust the angle of each subwoofer to my liking.

The mounts don't have to be really strong the fiberglass will be the strength

This is where that rabbet cut will come in handy, start wrapping the box leaving 8 to 10 inches between each staple keeping the fleece moderately tight.

Now we can pull the fleece even tighter making the fleece curve around the rings stapling while pulling it tight you are going to staple all the way around the box with no space between them.

Next mix up a good amount of resin and soak the fleece with a thick coat, you want the resin to soak all the way through so don't be afraid when applying the resin!

Let the resin cure and apply a second coat before the next step...

Unfortunately I'm missing some pictures of this step but its pretty strait forward.

This is where it gets messy i recommend getting some latex gloves and a mask!

Cut up squares of the fiberglass cloth and and mix up some resin, coat the inside of the box with resin in sections and dab the squares of the cloth onto the fleece.

If you want a really solid box with a deep sound coat the fiberglass with two coats of kitty hair, seen in the above picture.

"Kitty hair" is short strand fiberglass mixed with body filler is sold at Home Depot or your local auto body repair shop

Here comes the fun part...Sanding!

Sand the area where we applied the resin with 100 grit to knock down the bumps and when smooth as possible apply a thin coat of body filler to the entire area

Go back with 100 grit and take out the rough edges the body filler will leave behind and work your way up 400 grit.

Now spray the whole box with filler primer and sand with 600 grit until smooth.

After putting the box in the truck i decided i wanted to port it...

So i bought some ABS pipe and a hole saw set.

Since the box is already built we can measure the air space pretty easily by either measuring with a tape measure and doing some math or the easy way and fill the box with packing peanuts and take that amount and fill up a square cardboard box, this will give you an accurate and easy way to calculate the air space.

Use an online port calculator to determine the length of the port needed for your enclosure

I cut the ABS to length using a band saw and then drilled out a whole in the box with a hole saw the size of the inside diameter of the ABS, then i cut a ring to accommodate the outside diameter of the ABS pipe and it will butt up to the hole cut into the box (see picture above)

Last step!

You can paint it your favorite color or maybe wrap it in vinyl, carpet or both like pictured above...

Thanks for checking out my Instructable!