We recently did Robin Hood with the young players. This year the set was practically an open stage, just some painted rostra as props. One of them was to have a small campfire painted on it. So I got the set builders to cut the flames shape out of thin ply and space it away from fron the box, and we put some LED tape in the gap. A battery and an RC4 receiver feeding DMX into a cheap 6-channel ebay LED dimmer allowed the box to travel freely on stage on the revolve and be moved. A bunch of effects flickering the LEDs, and it worked surprisingly well.