Thank you for your comments gentlemen.
Funny you should mention exotic...
Attached is a pic of a (pbly big) bass trap from inside of it. A second pic is in a follow up post, which is a shot of the rear wall that it is exposed to. The opening is behind, and somewhat larger than the black fabric area. It is the full width of the room, about 4 feet high in the center, reducing to 2' at each side and is 13' deep.
About 5' behind the blue modular absorb wall are a pair of 6' doorwalls (back-to-back) for entry from a lounge/vocal/dub room that is 13' deep as well. The trap is above this room.
I have experimented with its 'operation' quite a bit. At one time or another, it's been filled with either fluffy fiberglass rolls, cellulose (still in the bags) or about 80 of those modular absorbers. They all worked quite well, and prevented a return to the main room. Will probably put the cellulose bags back in, as that was the cleanest, most adjustable filler. The only thing left to my imagination would be to build the worlds' deepest diffusor into it. How about that one?