Rich at ReptileBasics who is pretty much the expert at heating tubs with flexwatt recommends that no more than 1/3 to 1/4 of the inside tub floor dimension be covered by the flexwatt which must be controlled by either of t-stat or a rheostat. Here's his Flexwatt FAQ to learn more about this very stable and cost effective heating option.

http://www.reptilebasics.com/Flexwatt-FAQ-sp-4.html

Whatever heating option you choose you need to have a good way to read temps at the substrate level where your ball python spends the vast majority of it's time. It's also best if you can set up the enclosure a few days to a week in advance so you can tweak anything that needs fixed prior to the snake being introduced.