Jealous for sure... wish I had the talent to weld something like that!
As far as the oven type box goes - the more trapped surface area from the top of your box - the better heat you can get. I would focus on having an ability to vent more / less based on warming needs. I would imagine that with the right amount of control and sealing in the door you could cook a pie or cobbler in there easily. Additionally with good sealing I don't see why you couldn't go with more than one shelf in there to add more cook space.