hot air and smoke flow

[bbqguy]

I am building a custom built meat smoker, the heat flows into the main cook chamber from the fire box. Once in the cook chamber the heated smoke-air flows out the other side through an opening into the chimney. In the last smoker that I built with this design, the meat has a tendency to be more on the dry side. What will the effects be on increasing or decreasing the size of the entry points or exit points of the heated air. Is there any way to have less of a draft effect by the size of the entry and exit points? Is there anything to having the entry or exit points at different heights within the cook chamber? I appreciate any input that you can give me.