For that model, with the open bottom, the only useful mod I did was to add a grate in the charcoal pan. I went pretty simple, got a grate that would fit, then just set it on 4 golf ball sized wads of aluminum foil to keep it off the bottom. That allowed the fire to breathe, and in conjunction with the water pan, kept the smoker in the 225°-250° range. That smoker is actually designed to work as it was built. The loose fit between the lid and the body is the "chimney" and the open bottom is the intake. The water in the pan evens everything out. With the exception of the monumentally stupid design of the coal pan, it's actually a pretty solid design.
All that being said, however, the best thing I ever did with mine was to donate it to a rented beach house and make a mini WSM.
It was just too limited in its ability. It got really frustrating really quickly.