How to Build a Blacksmith Firepot by Don Meador
Build a moveable furnace on legs traditionally used by farriers. But you could use it as a small forge, heat treating, etc.
With simple tools you can build a fire pot 7 1/2" square internally and 4 3/4" deep, blown by a shop vac (here, at least). You can fire it with grocery store charcoal, coal, or even coke.
Simple, but useful project. Detailed instructions and many drawings. Good information.