Here is a much better alternative to outside insulation: An inside heat shield.
Basically, it's a stainless steel sheet metal lining that is seperated from the firebox via spacer rods.
You will probably need 2 pieces of stainless steel sheet metal each a little more than half the length of the fire box and each a little longer than the circumference.
First you need to weld or bolt (or whatever) several horizontal spacer rods running the inside length of the firebox. I'd say 4 to 6 rods in total.
Then you simply roll up each piece of stainless steel sheet metal, insert it and let it go. When it atempts to spring back flat, it should fit snugly against the spacer rods creating a dead air space between the stainless steel sheet metal and the fire box.
Then do the same for the other piece. And by making each peice a little more than half the length of the firebox, they should overlap slightly in the middle, thereby creating a complete heat shield.
Of course, the ends will not be shielded, but then they're not as critical a source of heat loss.
Comnprehende? If not, drop me a PM.
Dr. Mark (STL)
Ph.D. (honorary) Bovine $hitology
A thin line separates paranoia from an acute understanding of reality.