You could do a insulated fire box and upper barrel, at the time, i didnt have much money to play with at the time i was building it, if you could find a way to get it around the inside of the barrel, yes i think it would work.
It looks like you have the basic design going good, they are fun to cook on, and also a challenge, i have had my Lang now for about 2 weeks, and im not sure if i will ever cook on the barrel smoker again. Im going to take the volg's kit and build another double barrel unit, but this time it will be air tight, only openings in the door and flue, and i will use it as a shop heater, with the vent stubbing out the side wall of my shop.