The gracious curve of a sub box. A compact, space-efficient kick panel. Smoothly installed monitors in the center stack. All of these things commonly require the use of some resin, fiberglass mat and body filler - ingredients that many DIYers look upon with trepidation. To carve up some MDF board and create a basic enclosure, you'll only need some simple familiar tools like a jigsaw and drill. But just because working with fiberglass is foreign to you, doesn't mean it's difficult. On the contrary, it's quite simple when you know the steps, as this composite speaker enclosure how-to will teach you.
These are the basic ingredients you need when prepping a fiberglass project. Don't skimp on personal protection. When doing this sort of project you inhale a lot of styrene fumes and fiberglass dust. Always work in a well-ventilated area and use an organic vapor filter respirator. To reduce itch, wear latex gloves and protect your eyes while grinding and sanding fiberglass, and while pouring, mixing and applying resin. |  |  |
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For this demonstration, imagine this is a spare tire well. You can use the techniques for this particular build in tire wells, passenger foot wells, storage pockets, etc. | 
Protect the entire area with masking tape at least 3" to 6" beyond where you plan to lay fiberglass. I like to use 3M 233+ but regular 2020 or 2050 will work too. Smooth over any indented areas with additional tape and/or cardboard to make the fiberglass easier to apply and remove. Make sure that the shape doesn't have areas that may wedge in place once the fiberglass hardens. |
In addition to tape, protect your form with heavy-duty aluminum foil sprayed with 3M Super 77 spray glue. The foil creates a chemical and thermal barrier that the tape cannot provide. First, mold the un-glued foil over the shape. Then, pull it back up and spray it with glue. | 
Press the foil back onto your shape using a wooden roller to flatten the wrinkles. Round over the edges of the wooden roller with a belt sander first to prevent sharp edges from piercing the foil. Patch any tears with smaller pieces. | 
Place 1' to 2' of plastic sheeting around the area. For added protection, apply two light coats of mold release wax, Vaseline or PAM cooking spray to the foil unless the surface is vertical or inverted. Then use a permanent marker to draw on your footprint. |