Insulation for inside of smoker

If you have access to a welder, you can make a double wall barrel, with sand between the walls.  Great insulation, resistant to incredible temperatures.  It does not out-gas.  And provides stellar temperature stability.  Cut the ends off one barrel, slit it lengthwise.  Weld it to the outside of the barrel your grate will rest in.  Space the outer and inner wall by about 3/8 inch.  Fill with sand, weld shut.  It will be insanely heavy, a stout support frame will be required.