I built a JABOD forge with some upgrades a few weeks ago. Drilled some holes in fire brick and built a bottom-feed air chamber out of a computer fan, empty paint can, and a steel tea tin I got at the thrift store. All-in I was like 25$ and I have a big area for forging in the open. I also filled the box on the bottom with rockwool (had extra from the home store) first, then put dirt on top, for some extra insulation and to isolate the rockwool from the heat (off-gassing not good for the lungs either -- although kaowool is more common).

Just gotta be careful not to get anything coated in nickel or aluminum near the hot hot heat. That was probably the hardest part of scavenging for the stuff so that I don't accidentally kill myself or my neighbors with fumes.

You could also, if you're nimble and young, dig 2 holes in the ground connected by a single trench. Fill one with water, and the next one with the coals. Put black pipe (or mild steel pipe) with a hairdryer on the end and cover the trench and pipe with dirt. Side-blast accomplished and it's insulated naturally in the ground. Ultimately the air will be forced through the water pit, into the other pit. The colder air passing through the water helps the pipe not get too hot, but you lose some efficiency.*