Thanks for all your help, but this humidor will be nothing but an expensive paperweight.
I went to load it up last night, and mold was already there to greet me. Here are some photos. I seasoned the box with the shelf inside. There was a glass dessert bowl with a soaked washcloth in it, directly underneath the shelf. The shelf had mold on it.
When I had the first bout of mold, it was when I was using the humistat on the humidor lid, and behind the humistat was mold on and around the magnet. I'd cleaned it with iso alcohol, but didn't seem to help. There was a small bowl of dist. water on top of the shelf.
Guess I'll be using the tupperware permanently. What a shame!