Are you looking for a method to increase the efficiency of your offset smoker? Want to use less fuel and hold more consistent cooking temperatures? Ceramic insulation may provide an answer.
In my upright smoker, I have 1 inch of insulation around the entire box -- including the cooking chamber. It really works wonders.
Why not insulate the firebox for your offset smoker? I'm not a welder or fabricator, but I'd bet it's possible to modify the existing firebox with some insulation to improve heat retention.