Jim, I've been pondering a mobile oven for a while myself & had posted this sketch for a steel frame a week or so back for someone else interested in a portable.
This would take the place of any masonry hearth as FB board/cal-sil would go right over this frame after it was skinned with steel. The sketch is just the heavy steel channel with lighter angle or channel to support the center of the floor.
I think it would weigh around 450-500 lbs ready for insulation.
Ted