[replicant_argent]

You could poly it, then use some spanish to line it a bit. Weatherstripping is cheap. If it is relatively solid, and the joinery doesn't move much, you can seal the existing (probably aromatic) cedar with something that will provide a moisture/odor barrier, then maybe silicone the joints where movement is likely to occur. This would give you a "shell" to work with that shouldn't leak much humidity. Then to the seal on the door. Some rare earth magnets, coupled with adjusting the door for a tight fit and some thin open cel foam might do the trick.
Of course I haven't tackled something like that before, but it sounds like a good plan.
It sure would be a pretty humi when done.