Belly heat is NOT required for digestion this is a myth. Correct temps are enough.
The tank you have can be made to work and reasonably well. A custom enclosure would be the best option IF you can afford it. I might suggest a different one. Side turning the tank. This will allow the use of a UTH (not for digestion but simplicity it is easier to control) The closed top holds heat and humidity better so it solves that the only problem is ambient air temps and building the door.
http://ball-pythons.net/forums/showt...arium-Solution
Something for you to think about. I have two of these in use currently and once the ambient air temps are over come I like it better than most other options I perfer this to my vision enclosure, and it is a toss up over the sly plastics I have. I like my custom plastic ones best then either the tanks and sly equally with the vision slightly behind. I am still not certain but the point is they are very close to a custom system.