I got ambitious (for me) and created some burned buildings. Ten altogether. So far, this scenario requires six. The blog page is updated to show the burned buildings.
Here's how I went about it:
It was fun, because it was quick and easy: from Michael's, a three pack of paper mache boxes, equals six square buildings in three sizes (well, technically six sizes, as the lids are a tad bigger than the boxes themselves; I can't see the difference with the naked eye); and a four pack of wooden rectangular boxes did the rest. Using my bag of black gravel and various pieces of wood and bits of crap that I saw during the project, I tossed it in and arranged everything to look like the last not-quite-consumed timbers of the inner structure.
Trouble came when I went to spray everything black: the light bits of wood and trash moved! Duh! I resumed the spraying at a distance where the "whoosh" of air would not blow everything out of arrangement, at least not too much. That made a sticky surface, then I closed in for the really thick spray. Mostly it worked. I used up the last of a can of Krylon black primer, probably no more than c. ten percent full. Then I doused everything down with a full can.
I don't know how robust the bits inside the ruins will be. Probably I will have to glue pieces back in. I should have glued them down in the first place, then sprinkled my black gravel on last, then gone to town (punny) with the spray can in complete confidence. But I was cutting corners, trying for absolute speed; and I succeeded in that at least. At no point did I have to wait between stages in the assembly and final spraying.
Oh, well, better system next time; except there probably won't be a next time. You'd think that, by now, at the tender age of five-plus decades of model building, that I would have taken into account the forced air that comes with the spray paint! Yes, you would think that. Oh WELL!