I tried, and gave up after about 40 hours. Not because I couldn't do it, but I reached the point where I decided it wasn't worth it to continue building a box that in the end wouldn't even sound as good, or necessarily look as good as an mdf box. So I cut my losses (about $200), and went back to mdf. If you are going more for the look of your install, then fiberglass might be worth the huge ammount of extra time, money, horrible mess, and pain in the a$*. If you're more concerned with sq or you're shooting for a specific volume, definately just build one out of mdf.