Recently on either New Yankee or Old House Classics, there was a segement on building your own stem bending rig.
The basics were:
A propane fired burner that heated water in a BRAND NEW gas can. The nozzle of this can was piped to a long 8' ? PVC pipe. The pipe looked to be 6" in diameter. (Available any home center) One end was sealed with PVC cap. Glued I assume. The other closed with a cap (same pvc end cap) , but with a rope handle to pull the cap off to put wood in or take wood out. On the same end there was some steam relief opening on the bottom of the pipe. As I recall it was substantial in size , maybe 2" diameter.
The pipe rested on a wooden frame affair, built from 2x4's.
Please note I have no experience in this matter, but would pass this on with the following notes of caution.
Rig must be outside Make sure the relief opening is large enough Use only seasoned wood, I can't imagine green wood working. I think he cooked the wood for 20-30 minutes? Quickly pulled it out of the cooker and bent into shape on a stoutly built jig, using large pegs and wedges.