When I saw the competition criteria, I knew I had to enter.
The idea for this entry is a last-ditch covered "bunker" that's based mainly on the top of a Chimera APC that's been ripped off the rest of the chassis during an explosion, and thanks to some half-assed welding by the field repair crew, remains relatively intact. It forms the roof, with I-beams, H-columns, and perhaps a ruined building girder holding the thing up, with the usual, and some unusual battlefield detritus forming the walls, perhaps with some spikes made with sheared and/or twisted pieces of sheet metal sharpened to a nasty point, and some small bits of razor wire arranged to form a continuous line, more or less.
I might not work on this much until after GenCon though...