Sub Box layer question?

Ok so I never finished my first set up because I wasn't happy with the way it looked. Now I have decided to do a mostly fiberglass box for my subs. I am going to be running 2 JL Audio 18w6's and 2 or 3 JL Audio 12w6's. Leaning more to just the 2 12's for sound quality reasons. The question I have is how many layers or how thick should I lay with the fiberglass? The specs on the 18's say to use at least 1" MDF so I can't imagine how thick the fiberglass would need to be. I am going to start by building my framework, using speaker rings and MDF and then stretch carpet over everything. Plan on getting that all done then taking it out of my vehicle and laying multiple layers on the inside of the box. My other thought would be to get a fiberglass chopper gun and do it that way. Just not sure which way would work best. Any ideas would be wonderful. By the way this is all in my 2000 Suburban.

Come on guys, someone has got to know the answer to this. I really would like to hear everyones input.

everyone on the fiberglass forums sleep like crazy.........ive heard in the past that for fiberglass the reccomended is about a 1/4 inch........the biggest thing you want to make sure of is that the box does not flex in any way........after every few layers that you let cure......check to see if you can flex it in any way.....flexing always equals lost sound.......i have about 5-8 layers in my box right now and parts of it still flex.....so im still layin glass in there......so yea recommended is usually about 1/4 inch all the way up to 1/2 inch