if there is wood handy: Build a wood-fired stock tank heater - Backwoods Home Magazine
the flaw with this guys design is the stack needs to be much taller than the inlet, and if you insulate the stack it will draw much better, as in just weld another pipe around the outside of the stack and fill it with ashes, when it is full cap it so rain cant get in but leave a vent hole to prevent steam pressure
Now that is an interesting idea. We have plenty of wood!