I have been looking at the Outback oven and they use an insulating material to help hold the heat in. One thing about this flashing oven is it does take a while to cook things and I think it is because of heat loss.
I have a friend that is a fire chief for a small town fire dept. They are getting new turn out gear (fire proof jackets) and I asked if I could have one of the old ones. I was thinking about tearing it apart and making a cover for the oven. I would have to find some Nomax thread to sew it and it will probably be heavier that I want but if I get it done I will let you know if it works.