Because the resin used in working with fiberglass can be dangerous if inhaled or misused, you should always wear gloves, goggles and a respirator when working with it.
Measure the area you'll be putting the enclosure into to make sure you'll have enough room to make your box large enough for the subwoofer.
Before you begin, you'll want to make a mold for your custom subwoofer enclosure. You can do this by stretching painter's tape over the area you'll be installing your enclosure into, layering the strips so that their seams overlap. If there are any areas that seem weaker, you can use duct tape to reinforce the mold. Then, lay painter's drop cloth over all of the surrounding areas, because you'll be working with the resin here initially.
Knytex cloth is the material you'll use to absorb the resin. Lay strips of knytex over your tape mold, making sure it is smooth. Then, remove the knytex and spray a generous coating of cooking spray over the tape so that when you're finished with the mold, you can remove it from the area. Return the knytex to the area so that you can begin.
Mix your resin according to the package instructions, then carefully brush a thin layer onto the knytex cloth, letting it soak in. Let the resin cure for about two hours, then check it for tackiness. If the resin is fairly dry, you can remove the mold from your space.
Once you have the shell of the box, it's time to apply more layers of resin for adequate thickness. However, remember to always add layers on the inside of the shell, because if you add them to the outside, the box will no longer fit in the area you made it for. To add layers, you'll add pieces of knytex and resin, repeating the steps necessary to make the shell. You'll need about four layers on top of the shell to make the box thick enough to house the speaker.
Once your fiberglass is thick enough, it's time to make what will become the top of the box. You can do this using some MDF. Use a router saw to cut a circular piece out of the middle of the MDF that is the same size as your speaker. This is what your speaker will mount to, so that its face will stick out of the box. Lay the MDF over your fiberglass shell, leaving your speaker out, so that you now have a complete box with a hole in the front.
Next, staple a piece of stretchy fabric that is large enough to cover the MDF to the sides of your fiberglass box. Apply a generous layer of resin to the fabric and allow it to cure overnight. When the resin is cured, you can file down any rough edges, trim excess fabric, and cut a hole in the fabric where the speaker will mount.
To finish the box, you can use spray adhesive to cover the front, and sides if you wish, with carpet. Then, mount your speaker into the hole you left in the front of the box. Now you're ready to place your custom fiberglass subwoofer enclosure into the space you made it for.