@musicjones - take a really good look at the Owens 703 FRK. I have used this extensively as well as the Roxul Rockboard 80. You may end up having better bass absorption with the FRK due to the foil membrane (and it will not absorb the upper mids/highs as much). You could probably layer a 2" 703 FRK with a 1"/2" standard 703 to get a 3-4" panel.
If you really want an effective bass trap, look at the GIK Monster Bass Trap with the FlexRange Limitor. This costs money, but the membrane panel does an excellent job of capturing 60-80 hz and also does not absorb the high frequencies (which can suck the life out of a room).