keg insulation

[TRANSPLANTED HILLBILLY]

hey guys, in an effort to increase efficiency I would like to insulate my boiler/ 7.75 galllon keg. I heat with a turkey fryer burner so I need flame resistance. I was thinking of getting some metal sheeting of some sort, building a shell out of that, and filling with mortar. This should work O.K. but sounds a bit heavy/cumbersome to move when not in use. I know that this should allow me to boil slower thus producing a better product. Does anyone have thoughts that mey help reduce weight? Maybe even something that would slip on/off. Any thoughts would be appreciated.