I have a fair deal of knowledge on fiberglass, take my advice build the sub enclosure out of MDF or wood, seal it then just make a fiber glass overlay that is Mearly cosmetic. 1) you can pretty much do whatever you want and not have to worry about proper size for the subs or rattling 2) if you are stuck on fiberglass go to homedepot get some great stuff insalation, line your backseat with plastic and tin foil and tape then spray it all over IN A PLASTIC BAG, its the best way to get a reverse mold and figure out correct demsions for a fiberglass box. Not to be a D!ck but it sounds like you are getting into something you are not ready for. Its not as easy as laying a few layers of fiber and creating a flat face for a sub, ITS all about demesions and thickness minnimum I would do 6 layers of fiber if I was going to build a complete box. So roughly (6) 6x6ft sheets of fiberglass woven or matt, plus 3 gallons of resin, 12 oz of hardner, your looking at spending about $500 that doesnt include things like tape, brushes, mixing trays, resporators, bondo, paint, sand paper, carbid bits for cutting and trimming, Dremmel, 12in sub rings for a rolled edge, THAT IS JUST TO NAME A FEW there is always more crap you need that you didnt think about. Not to deter you but I hate the how to's posted online making this look so easy when most of the time it is something that should be left to the professionals. Feel free to ask me any questions or check out some of my threads for ideas I wouldnt mind helping you through this I just want you to know what you are getting into be prepared to spend $1200 assuming you have no supplies. www.fiberglast.com is where I get all my stuff.