The other day I tried a mod on my Bandera offset that I had seen somewhere - I lowered the exhaust pipe down until it was at the same level as the grates. I guess the idea was that the heat would fill up more of the chamber before exiting. Anyway, as it turned out, I lowered the pipe below the upper level of the firebox. It had the effect of choking out the fire and producing a lot of bad, white smoke. I had to keep the firebox open to allow the fire to burn properly. I reversed the mod and the smoker was back to normal.
The reason I bring this up is that the wall of your cook chamber blocks most of the fire box - you may find that you don't get the proper air flow because of this. If that happens, the fix would simply be cutting the opening up higher.
Of course, I may also be completely wrong about everything.