You may use wall insulation fiberglass, it will work, but it does not work nearly as well as rigid Type 700 fiberglass boards. I won't use insulation fiberglass, as it will fall apart, which can lead to problems down the road. As for other materials, foam, polyfill, etc, all work equally well in the right proportion. The main advantage to Type 700 is its very high index of resistivity, which means less of it is required to achieve the same level of damping. There are instances where that might be significant, but for all practical purposes a couple of inches of any of the usual materials will accomplish the task at hand.
Bill Fitzmaurice Loudspeaker Design
The Laws of Physics aren't swayed by opinion.