I definitely think that 2-part foam is stiff enough. It seemed significantly harder than styrofoam, although not much denser. I squeezed the softball-sized chunk that was in the store, and I couldn't compress it at all. It sounds "hard" (and kinda hollow) when you rap on it with your knuckles. Neat stuff, I thought.
I just thought of a another wacky way to form your enclosure(s). It's sort of the inverse of your foam-in-a-bag idea. Take something like a beachball, insert it (deflated) into the area behind the 6x9 cutout. Then inflate it to your desired enclosure volume. Then, block off the area well aft of the cutout, maybe using the supplied piece of foam you described earlier. Finally, pour the expanding foam stuff "around" the beachball-like object. After the foam hardens, remove the beachball and enjoy your organically shaped, acoustically neutral, hermetically sealed enclosure!
Finding the right object to "pour around" would be the tricky part. That, and getting the foam to completely fill all voids without busting out (or into) anywhere you don't want it to. I think you'd need to pour from the very top, (that gravity thing you mentioned) and also allow any excess foam to escape the top in a controlled manner. I imagine you'd want to drill a "fill" hole up high on the panel. The foam I linked to can be poured in stages, I believe. I think as long as the area was not closed off while the foam was still expanding, the foam wouldn't exert much pressure on its surroundings. (But, I don't know for sure!)
A nice bonus is that your MINI will be unsinkable!
-Dave
