Bandera's lose a lot of heat out of the lid to the fire box and through the sides and bottom of the firebox. I lined mine with thin fire brick and sealed off the lid with a 1/2" plate of steel, it doesn't have to be that thick, but I'm using mine as a griddle. That fixed the problem.
"It's all done with smoke and mirrors..............I think I like smoke the best"
Highly modified Bandera w/ griddle on firebox