A 6" strip of flexwatt across the back of a 2 foot cage will make it difficult for a BP (especially one of any size) to thermoregulate. 2 feet worth of snake will be able to push up against the back wall, but what about the rest of the snake? Also, when you provide a hide for the snake, it will then not be able to warm itself against the wall that's providing the heat, resulting in no actual hot spot and a waste of flexwatt.

I'd re-think either your design or your choice of inhabitants.