Here are two quick examples that can be used. Nothing too hard to replicate once you understand it. Just open the levels up in JCS and stare at them for a while. There are probably other ways to do it, but I'm not going to bother.

BTW, the bomb scenery is there in the first example so that the TNT explodes instantly. Apparently ammo crates (like the seeker crate I used) don't fall down when the tile below them disappears. Gun crates seem to fall in that situation though.