well since my car is garaged for the winter i got a interesting idea of fiberglassing 3 10 or 12's in the trunk in a sort of U shape, like one on the left wall, one in the right wall and one in the center angled out, just an idea but really dont want to do it if it ends up being to heavy, right now i have a 12" with a pretty big and heavy wooden box

how heavy would the fiberglass enclosure be if it was made with an average thickness? obviously dont expect an exact weight but just ball parked comparing to something like the weight of a box made for duals? or is fiberglass pretty light

 

Fiberglass can be as heavy or as light as you like it. If you're going for only fiberglass, then you'll need 1/2" or thicker all over. That's pretty heavy. It's good for car audio as you really want your subwoofer to move air and move your butt at the same time (using the car as a resonator). A light weight box won't shake the car as much as a heavy one because it allows the box to move slightly.

If you're worried about weight, try using some MDF to build the enclosure(s), then covering it with fiberglass.

I'm working on molding a 3" reference speaker to my dashboard with fiberglass to complete my 5.1 surround sound system. Because of the size It will probly only need to be 1/4" fiberglass. I'll probly go overboard and make something that will outlast human kind though.