question for those that build fiber sub boxes

[stich626]

hello.. this isn't for a truck but.. a good cause
my monty SS it's an 86, now these cars in the trunk ,right behind the rear seat, have a shelf thats higher than the rest of the trunk floor..
I want to build a box that sit here and the drivers facing the seat .
I will be sealing the box face to the seat back cars framework(not the seat back itself) the rear deck(that on this car is tiny) and the shelf and the rearseat support frame crossbraces. as I really hate cars that boom boom boom just as loud in the trunk as the seating area..I want it all in the seating area, not the trunk(well as much as I can,HA)
I plan on cutting the seat back foam(junkyard seat back) and recovering the cloth. so that the sound doesn't have to fight to get through the foam..
now remembering "lite is good" I was thinking alum angle to build the basic frame(bones) then make fiberglass panels that cover that.. the shelf is 4'+ long would I have to brace the lenth so the fiber panel doesn't flex with the air movement, and if so, every foot, 18", whats a good standard, the box will only be 15" tall at the most (ballpark figure)
now the front panel that the drivers are screwed to..something tells me just fiberglass isn't gonna hold up all that long. without stress cracks and then failure.. mdf is heavy as hell.. would 1/4 plexy work, or would something else be better(chip board/plywood)?? then cover in fiberglass, tho, that is somewhat pointless as no ones gonna see it..
I say lite as this car will see solocross like events and drag raced at times and a box that is lite helps here, 1) it won't totally screw with the set up and 2) lite enough to take out for those times you want it out..

it will be a dual driver box.. may or maynot port the box. if ported it will port into the cars factory 4by10 deck holes or ?? a tuned port in the face fire'n at seat??
with te area each driver would have a sealed box would more than likely be best(and tighter).. but power hungry..


thanks for any input on this.. fill my gray mater with ideas..