How do you propose to make one from fiberglass? Are you going to make a mold first? It would be cheaper to glass over the mold (plywood box?) and use it as your sailbox. The top, bottom and sides of my sailbox are glassed with two layers over plywood. These overlap at the corners and edges giving 4 layers of strength. It was a messy job but will be worth it in the long run. Just be prepared to burn up a couple of sanders