you really could fill the cavity with 703, it just costs more and the fuzz (lower density glass) really works fine near the boundary...
Yes... the large room behind the other room with a sizable "port" cut between the two, and filled with fuzz would make an awesome bass trap... It could be tuned (by means of the port) if desired, or just openned up large enough to be "broadband">.... now, who wants to give up an entire other rooms for a bass trap.. that's my kinda client!!