The best way to make a fiberglass enclosure is to line the area you are working with aluminum foil and make a small mold in that. Then take it out of the car and keep applying layers! you want it to be atleast an inch thick. For the sub, build one side around a peice of half inch MDF so it has support when mouted.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>