take out the factory deck and the material that covers it
cut 6 1/2" rings with spacers to make them rise about 2 1/2 inches above the deck itself
lay fleece over the back deck(make sure to wrap it tight) then lay your fiberglass resin over the deck
you will proably need about 4 to 5 coats.
after you are finished with that sand it down and make it smooth the lay one 1/4" thick layer of bondo over the deck to fill in all of the imperfections.
after it hardens sand it down again, until everything it completely smooth
then lay either the carpet or vinyle over the deck to and screw the speakers of choice into place..
here a few pictures of a box that i designed and built for you to go by, this is something very close to what you will be doing....
good luck and let me know if you need any other help