The spot that has my water pan is the stock location for the Pro-Q smoker. The charcoal extends to the top of the bottom bowl that has the legs on it and the first "stacker" has the water pan and grill. Additional stackers would have more room and then the top.
With 3 stackers and no waterpan I can emulate one of those 50 Gallon "Drum" smokers.
When they get in stock at the local stores I'll try a BGE 18" pizza stone instead of the foiled water pan. If that works I can make a 6" taller ring for the charcoal basket and get really long overnight cooks.