Okay where's my soapbox!
Some observations...
1) heat naturally transfers to the gas and solids around it.
2) heat can't be stopped but it can be slowed down (insulation) and turned SLIGHTLY (from straight up to slightly over).
3) heat rises NATURALLY not because it has more energy but because the surrounding gas (atmosphere) is heavier (oldest debate in the field of thermodynamics).
The goal of a smoker design is to create a device (using the above observations) that even a good ole boy that's been in the apple pie can operate. A device that allows a basic fire to NATURALLY heat enough hot space to cook some food without having to fight the apparatus to get to and stay at temp long enough to complete the job - and looks GREAT parked in the driveway. But most importantly - IT IS REPEATABLE. Imagine having to relearn your smoker every time you use it... And all this without breaking the bank to cover the fuel bill.
The best smoker design will have the draft at the very bottom where the oxygen-rich heavy air is
- then the ash pan to keep hot coals from falling through the draft opening
- then a drop zone for ash/coals to fall through and draft air to flow to the bottom of the fire basket to provide combustion air. Everything up to this point is driven by the hot expanding air ABOVE the fire trying to get out the Stack and a well sealed cabinet.
Once combustion happens and the air has been stripped of it's usable oxygen it continues to rise rapidly but it's squeezed by the transfer opening (kinda like putting your thumb over the end of the garden hose) This has to be the right size to reach equilibrium between what heat the firebox has to offer the cook chamber and what the stack/draft combination will allow for a flow rate. Building just enough fire to do this makes finding this equilibrium much easier.
Once the heat expands the atmosphere into the cook chamber (about 1:3 ratio) the stack takes over and keeps a VERY small negative pressure on the whole exchange.
IMHO - center the fire basket top to bottom and side to side in the firebox and feed the fresh air into the fire box directly OPPOSITE the transfer opening and as low as you can get it. This will allow the firebox to produce the largest possible quantity of hot air all around the basket. As you observe how small of a fire you can get by with then TUNE your basket in from all directions so you have a central basket that holds the correct size fire PLUS one offering of new fuel. Keep in mind in your design that you may want to slightly enlarge this central basket in the winter time depending on where you live.
Keep a log of your cooks and I think you'll find that in the long run it will be the most consistent and stable for temp - 2 things that are typically the hardest to attain anyway and solve the other issues through other means...
MAN - I haven't talked that much since the wife discovered that new sports car parked in my side of the garage....
But I feel better ...
I just used crayons, rick broke out the physics book 
