I also recommend a Radiant Heat Panel from Pro Products. However, I definitely DO NOT recommend an exterior polyurethane for the inside of a snake cage. Unless you mean one of the few waterborne exterior urethanes, then I think it would be okay. But even then you'd want to use an interior product. The exterior products have to have UV inhibitors added and be more flexible to deal with the exterior environment. I know it seems counter-intuitive but this actually makes them less water resistant, which is what you really need in a snake cage.
I can go into this more if you like, but I may not have internet access until this weekend.
Whatever you do, try to avoid using an oil based polyurethane on the inside of a snake cage.
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Current snakes:
0.0.1 Gonyosoma oxycephala - Java locale (green)
0.2 Gonyosoma oxycephala - Malaysian locale (green)
1.2 Gonyosoma oxycephala - Java local (green)
2.2 Gonyosoma janseni - Seleyar locale (all black)
1.2 Gonyosoma janseni - Celebes locale (Black & Tan)