Fireproof rigid indistructable board
is there any way to insulate outside the metal, to keep the insulation out of the smoke/moisture inside the cooker? that would also give you more options for insulation. most insulation (readily available) will get crud on the food. roxul makes a very high temp insulation, but you'd want it completely encased, as i understand it is nasty. also, i think their regular wall insulation will take those temps no sweat, but, again, on the outside and encased. for a gasket, any plumbing supply place should have boiler door gasket material. whatcha cookin'?