As you can see, I finished assembling my charcoal forge.
The construction is pretty simple. The firepot is a truck brake drum. A floor flange is bolted to the bottom of the drum. Not shown is a tee fitting; there's a 4" nipple below the tee, with a cap over it, to collect ash. Then there's a 12" nipple, which ends in the fancy looking thing with the red handle. That's a gate valve, which was twenty-something dollars, but worth every penny. The valve connects to a piece of PVC, which pulls airflow from the fan.
The fan is a wood stove fan that I got from Surplus Center for about $40, shipping included. The frame for the fan is just plywood and construction lumber, nothing fancy.
The forge works really well. I haven't yet been able to get it up to welding heat, which is around 2000°, but forging/hardening/annealing heat, typically 1400°-1500°, was no problem. But I need practice. My first effort was moderately successful. I tried to harden a plane iron and a chisel. The chisel turned out well, but the plane iron, not so much. This is to be expected; as Ron Hock pointed out to me, if it were easy, everyone would do it.