The doors will be more like 5'6" tall by 8' wide. I'm now wondering if bottom-hinged doors, doubling as ramps might not be a bad idea.

Or maybe I'll just make them removable so I can load/unload with a set of forks?

Inside the kiln, I'll use a sheet material. Outside the framing, I'll apply 3.5" of rigid polyiso, then wrap that with 1x4 strapping, then probably metal siding.

Still in the design phase!! Thank you for your comments and questions!