Door is 3 full sized insulating firebrick, in an angle-iron welded frame, hinged at the bottom. The door fits close, but it's not air tight. I need about 4000 watts to get this thing up to 1000 F in about an hour. Output from this particular controller is 2 volt DC, which trips a solid state relay, which trips a contactor. Contactor came from a scrapped electric water heater.
Gas is the way to go for larger sizes.
I just found a brush-torch, propane, used by farmers to walk along and burn dry grass. It is a venturi type burner, but it sure roars. The sheetrock will shrink, and if over heated, probably fall to a powder. It cracked randomly, but is supported on the outside by the steel box, and within by the brick. The strength here is in the steel box and the brick.