Here's the new forge I just built out of an old propane tank. After ensuring it was truly empty, I removed the valve (NOT easy) and filled it with water to displace any remaining fumes. I then cut the opening in the top, and cut off the bottom:
I used a hole saw to cut a hole in the top, for the access tube of the burner (made from a piece of black iron pipe nipple). I also attached toggle hasps to the sides and back (formerly the bottom of the propane tank); This allows me to remove the back for long stock, or to replace the liner:
I then lined the forge with a 2" layer of ceramic wool, which was then painted with 2 coats of Plistix, an IR-reflective refractory coating:
Here's the finished product, with some soft firebricks as the floor and porch:
It works well! I re-used the burner from my smaller forge, and the new forge appears to be more efficient; it seems to heat up faster, and requires less gas to maintain heat:
