I don't work with pieces quite that large, so this may not be applicable and this is with Kydex. When I have a piece too big for my full size oven, I just turn the top element on low and keep rotating the pieces. The ends cool a bit, but it's worked for 24" pieces or so. Make sure you've got a piece of sheet steel or something in there though. Don't put it on the rack.
You could make a simple solar oven. I'm guessing that would get it hot enough.