I own and use one of these exact cages with a 6ft Jungle Carpet inside... Mine is heated with a 60W Pro-Products panel that I had on hand to install myself. This setup works well for arboreals, but I tend to think it is too tall/arboreal-style for a BP. Try a shorter cage with flex-watt belly heat as mentioned above. Works great for my boas and big python in their terrestrial pvc-constructed cage.

Here's a sloppy phone shot of my CP setup just for fun: