I have more or less done this, but not with ball pythons. I have geckos and frogs in exo-terras with custom, plastic lids. The exo-terra lids are kind of garbage in my opinion. The only additional issue with a ball python may be that you'll nee additional lid security as a ball python is much stronger animal. However, the locks that come on the cage are pretty good if you size the lid right.
What I did was get a 3/8" piece of polyethylene (black plastic) and have it cut to size with notches for the cable holes in the back and the two raised areas in the front. 3/8" is about the thickness you want for the cages existing clips to be useful. I then cut a ~ 7" x 22" hole for the addition of stainless mesh (this assumes a 24" x 18" x 24" exo-terra) and siliconed it in place. You will likely have to screw yours down or bolt it into place given a ball pythons strength. I then attached a RHP (10" x 11") in the back with a thermostat probe dangling next to it.
This setup allowed for the use of an LED light and heating element, your needs may vary. I will note that I think plastic enclosures such as AP or constrictors NW are superior setups for pretty much any snake species. The exo-terras are really just nice for cages where you want a very aesthetic, planted enclosure. They strike me as ideal for dart frogs and arboreal geckos.