Below are many links, which describe the fiberglassing process while stepping through some actual projects. Some of these projects are for custom-fit applications, while others are not. Many of them do not apply directly to Mazdas, but I came across new ideas after reading each of them.
Link 1 - Protege 5 Project (good materials list)
Link 2 - Sound Domain - Kick Panels and much much more
Link 3 - Car Domain Kick Panel Project (basically just pics)
Link 4 - Random Site with many projects (mostly trucks)
Link 5 - Car Domain - Dash, Glovebox, and Trunk work
Link 6 - STi Sub enclosure - just pics
Link 7 - Large PDF from JL Audio, another cool enclosure
Link 8 - Good writeup with materials list and pictures
NOTE: You can get large pieces of dense foam, and piece them together with hot glue to form a cube, then you can shape the cube with a utility knife. This will give you a 'mold' that you can now coat with fiberglass. You coat the foam with tape and foil as you see in some of the projects. The difference is, you can cut the foam and pull it out when your piece is fully fiberglassed (assuming your item or enclosure has a hole in it somewhere). Your entire piece will then be made of fiberglass, rather than a wood frame with fiberglass on the outside.