You can safely completely ignore anyone telling you Ball Pythons need belly heat. They are a nocturnal snake found in termite mounds and burrows, not sunning themselves on racks and roads.
Belly heat will not hurt them if it is provided safely but, remember there are breeders that us only ambient temperature for their BP's. Personally, I prefer to keep a gradient and let my snake decide how warm or cool they want to be but, obviously my method is not the only way to take care of these animals.
As for sliding doors being too large. Could you frame in smaller windows? You might even be able to pick up sliding basement windows with latches to prevent escapes quite cheaply.