Fake, Material based holes

I’ve got an actor that I use to spawn monsters into the game. It looks like this:

222527-mob-emitter.png

That’s supposed to be a hole, though right now it’s literally just an indent with a flat black material on it. Is there a way I could make this look like a real physical hole without actually changing the geometry, loony toons style? The actor is placed on multiplayer maps in arbitrary locations, and might even disappear based on the game settings so it really just needs to be faked with a material trick.