Wood burning stove made from a gas bottle.
This has a flue that goes out of the side of the caravan, and an air intake pipe at the base that also goes outside - complete with a flap valve for damping. It's on legs and has a flat hotplate on top, and there are baffles inside to stop the heat going straight up the flue (which, incidentally, is made from the cover of a patio heater gas bottle, which we found thrown in a hedge - free metal, thankyouverymuch!).
The door is a pyrex dish.
It's brilliant - heats the van up from freezing to "oven" in about 10 minutes (don't worry, the caravan won't go anywhere these days, it's just for fun).