Take a gallon paint can (exact size not important - experiment).

Cut holes in sides of can as below:

Chuck anything combustible in and light it up.

The 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.

Put 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.

These 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!