You can cover any custom box with a fiberglass coating.
A fiberglass box has a wood frame under the fiberglass, so the box is not completely made from fiberglass. Since fiberglass is weak on its own, the wood is necessary to stabilize the fiberglass. Fiberglass can add an additional layer of strength to a box, and it also is easier to mold into unusual shapes and smooth edges for any box. The best base material for a fiberglass box is plywood, but you can also use fiberboard or even cardboard as the base material.
Instructions
1. Create a frame for the fiberglass box by cutting five pieces of equally-sized pieces from the plywood. Use the measuring tape and saw to measure and cut. Fit the five pieces of wood together to make a cube shape that has five sides. Attach all sides with screws and a power screwdriver. Larger boxes will require more screws, and L-brackets can stabilize the box more than screws alone.
2. Cut the fleece with scissors, in one piece large enough to drape over the entire box. Staple the fleece to the box. Start stapling the box in the middle, and stretch the fabric out toward the edges to prevent bubbles and wrinkles in the fabric. The fleece should sit as tightly against the box as possible.
3. Cover the box in fiberglass resin by applying the resin in a thick coat, with a paintbrush. Wear a respirator mask to avoid inhalation of fiberglass. Soak the fleece as much as possible to ensure that the box is coated evenly with the resin. Allow the resin to harden for two hours.
4. Attach an 80-grit piece of sandpaper to the sanding wheel. Sand the resin lightly. If any resin is built up near the bottom edges of the box, sand that area until it matches the thickness of the rest of the box.
5. Coat the entire box with another layer of resin. Wear work gloves to protect yourself from fiberglass splinters. Cut fiberglass mat pieces into large squares with scissors, and lay the matting over the resin. Overlap the edges of each mat square by 1/4-inch. Press the fiberglass sheets into the resin with your fingers or a paint roller. Coat the matting with another layer of resin. Allow the resin to harden for one hour.
6. Coat the box in a total of five to 10 layers of fiberglass. The more layers of fiberglass you use, the stronger the box will be. Allow each coat to dry for one to two hours.
7. Sand the fiberglass with 80-grit sandpaper. Cover the box in one final coating of resin to fill in any remaining rough areas. Allow the entire box to dry for 24 hours after the final coating.