If you can get ahold of some sheets of PVC 3/8" - 1/2" thick that would be best. Not only will PVC deal with the humidity better than just about anything but it will also let you put the flexwatt on the outside of the cage.
Check out this thread on stackable PVC cages made by a company called Animal Plastics. It might give you some ideas:
http://ball-pythons.net/forums/showt...With-Pictures!
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