My brother in law used an old wood fired water boiler. He chopped the top and bottom off of the boiler, which was made from very thick steel, and removed the central chimney aswell. He made it so that the top would go back on and placed rods through the top of the cylinder to hang fish and meat from.
When he used it, he set a small fire in the burner at the bottom to get the temperature up, this he could read as the old temperature guage was still fitted. When it reached a certain temp., he knew it was time to put the fish in and then he'd keep it at that heat for a little while. Once he'd left it five minutes or so, he covered the fire with sawdust and put the top of the boiler back on. This had a large butterfly valve welded to it, so after a while he opens it to let a rush of air go through and to keep the sawdust smoldering. If it fires up too much, he closes the valve off and thows a bit more saw dust on.
The fish I had from it was a freshly caught trout, and it was lovely!!