a gallon paint can (exact size not important - experiment).
holes in sides of can as below:
anything combustible in and light it up.
air flows in the bottom hole, and the aerodynamics of the can suck the exhaust
out of the top holes. This forms a vicious through draught, and the fire burns
almost white hot in a strong wind! It also uses very little fuel. Fire temperature
can be controlled be turning can away from the wind a bit, or by obstructing
the inlet hole.
a pan directly on the top of the can without the can lid (unless you are very
fussy about carbon deposits on your pan) for maximum heat transfer. Or to bake,
put lid on can, and baking receptacle on lid. Alternatively use a large lidded
tin, like a biscuit tin, directly on the can, supporting the item to be baked
above the bottom of the tin, so it bakes in the distributed temperature of
the tin rather than the direct heat on the base.
things are unbelievably effective, dirt-cheap and fun to make. You can easily
make one on the fly with a Swiss Army knife and a tin you pick up. You will
probably find yourself making lots of different designs just for the hell of
it!! They are also useful in that you can burn up your camp rubbish to cook
your next meal!