I saw such an oven posted a year or two ago... Guy used the blue insulating foam. As I recall it, the sides and top were full length, but the ends were foldable in the the middle through the use of duct tape hinges. You popped the ends out and you had the frame. Top was separate with the light board on it and bottom was two pieces of foam, one to the outside dimension and one to the inside. You opened out the frame, fitted it onto the bottom, but the bow in, set the top on. (Might have had a double foam layer there, too, like the bottom, for sealing) and you were in business. When done, top and bottom off, sides folded together and put it away in the rafters or wherever. I thought it was pretty neat and wished I had seen it before I built my big plywood one.