Fiberglass Subwoofer Box

[91OBSChevy]

I've been wanting to make a subwoofer box for my 91 c1500 single cab for a while now, out of fiberglass but haven't seen any on a 88-98. I've done a lot of research on this over the past few days but I don't know if I should do mdf and fiberglass or just fiberglass only. If I do only fiberglass, how much will I need for it to be strong enough to hold 2 12's and so it won't sound like crap. If anybody's done this on an obs or on another single cab, I need some advice.