Dave, you definitely have some mechanical background as descriptive as you have been with your suggestions.
Would I need to add a floor register to both sides of the smokehouse or is one side adequate? Folks have put registers on opposite side and opposing corners, to get some sort of "cross flow" to try and avoid linear air flow patterns... Use the remaining sides for upper air vents if you use them... I don't have any results to verify that install pattern.. but, I'd use it...
Would I need to put some sort of damper on the black pipe to adjust the air inlet? If you put a short piece of black iron pipe in the top of the door, it doesn't need any adjuster on it with your smokehouse... Continuous air flow, even while the smoker is not in use, will help to keep the mold from growing.... Too much air is almost impossible, when smoking meats, for a great finished product...
If I close the slide gate on the top of the smokehouse, the temperature goes up and I presume that's because the heat has no place to go.
I can completely close the door to the fire box and the lower air inlet and it still seems to stay burning fairly well. Its really easy to maintain temperatures from 275 - 300. Sounds as if there are air leaks in your firebox... I'm guessing, they are around the door.. possibly the flange or the door itself... Installing the vents on the smokehouse, and opening those, should slow down or stop the chimney effect from the smoke house body... that should reduce the "suck" and the fire should get under control... IF NOT... seal the door and flange to get better fire control... Having a sealed system allows for great fire control... tramp air can make temp/fire control impossible or difficult at best... The upper air inlet on the door will also help with fire control... reducing the "suck" in the firebox and allowing air in to the FB above the fuel....
I still have some learning to do with finding a balance between heat and smoke, but I'm still able to make some of the best tasting food ever!
I just finished smoking a turkey for the first time and it came out wonderful!
Thanks for the suggestions Dave. Any suggestions to help control air flow are very welcome!