Roxul Safe'nSound is a perfect material, reasonably priced, designed to absorb sound, and doesn't have the itching of fiberglass. In my experience it comes in just shy of 4" thickness, 48"x18" in order to fit in between studs. The cheapest down and dirty bass trap is to simply put a package in each corner. The plastic packaging reflects high frequencies and the bass frequencies go right through. However it's more fun and astheticaly pleasing to build an enclosure. I didn't cut the material, just designed an eighteen inch wide frame, used four layers to make a deep enough enclosure to be effective at low frequencies, and stacked two in each corner to have floor to ceiling coverage. A little chicken wire or course hardware cloth stapled to the frame is all you need to keep the layers stable, they'll almost stand on their own. Cover with your choice of fabric and you're done, which fabric doesn't matter at these frequencies.