I am planning on doing a set up in my trunk with a single 12" sub in the center where my spare tire currently sits. I want to glass the whole bottom and do two amps one on each side of the amp. Then cover the entire thing with a false floor. When building boxes and frames how does the actual MDF attach to the fiberglass?

 

Magic.

 

Also If you apply wet fiberglass to the mdf it will stick

 

Plain old fleece fabric. Build your general setup of the box, cover the "frame" with the fleece, staple on the underside of the frame. Slather on the resin/hardener mix, layer of fiberglass, another layer of resin. Voila

 

http://www.gadrivers.net/~dwicker/fiberglass.html