Thanks for the input guys. You have confirmed my suspicions on 2 fronts. The first being that the goal is to control temperature and humidity, as opposed to doing that while also maintains a closed volume of air (in other words, if the entire world were 65 degrees and 65% humidity, there would be no need for humidors) . The second being that regulating temperature and humidity (the actual goal) is facilitated by sealing the air inside the humidor (though limiting the total air volume is not an end in itself).
I will attempt to improve the seal on the humidor. However, as I mentioned in the original post, I'm not sure it will be successful.
The issue I'm having with the seal is that the door is 6 feet tall, and it has a lock in the middle and a magnet toward the top and another toward the bottom. Where the lock is, as well as various points along the 6' length, the seal is tight. However, at other spots, the wood is warped and the seal is not tight. Applying a uniform thickness of sealing foam or other material may not work, as it will push the areas that are already sealed apart slightly (they will still be sealed as the gap is filled by the foam) but that will cause a corresponding increase in the distance between the frame and the door at all points. Unless the foam is very easily compressed, this will not significantly change the gap in the areas that are poorly sealed. Even if it does, I fear the magnets that are located toward the top and bottom of the door, will no longer be effective at pulling the ends of the door into the frame (due to the increased gap caused by the foam.). My thinking on this is based on trying to use weather stripping for doors. Despite purchasing the thinnest weather strip I could find, the results were as described above. Foam may perform better, so I will give it a try and report back.
Either way, thanks again for the input.