Hi!
You can use heat rope, just hook it up to a rheostat and line part of the corners with the rope. PCPC sells it.
If you are using bulbs, you can make a box of metal mesh to bolt around the bulb (
http://www.ssnakess.com/forums/showt...&threadid=9794 theres a pic there). Or you can cut hole in the wood, cover it with mesh and put a bulb on the outside of the cage pointing.
You can also use heat tape, but I don't know if thats good for a cage like that.
Zoe