Pete, Great build. I have been watching for a while. I don't know exactly what dimensions you have, in saying that, you mentioned needing a draft for cold smoking and dehydrating.
If you add a chimney that extends outside, and it is sealed to the inside chamber, there is a law of physics (don't remember exactly) that at about 12 ft. (that includes from the bottom of the smoker,where you have an inlet vent, to the top of the chimney, there will be a natural draft. That draft is induced due to the difference of air pressure, (12' vs 0').
Old barns were constructed with slats in the floor and those peaks on the roof? That was for draft to dry the hay etc. and keep air circulating to keep stuff cool. If you are so inclined, try it. If not enough draft, add a length of pipe. Same principle that makes a chimney work. I would try 8" pipe and make it removeable when not in use.