How to heat this cage????

[AnniesMom]

http://www.arbreptiles.com/cages/dcage11.html

OK, this is an awesome cage, but how would I heat it? I would prefer to avoid heat tape if I can, out of fear. It is a double cage, with the top cage and bottom being seperated by 3/4" plywood. Basically a big box split in two. The bottom section can be heated with a heating pad under the cage, attached underneath. Lots of air flow under cage as it is off the ground. But what about the upper cage? Any heating attached to the bottom of the upper cage is exposed to the lower cage. HELP????