Picking the correct heat source for my enclosure.

Hi everyone I am adopting a 2 year old yellow belly ball python. (Her name is Emma) I just ordered one of the proline enclosures from Ed at Constrictors NW and I'm trying to figure out the best way to heat the cage. Since it is a smallish cage (36 x 23 x 18) I think a single RHP should be enough to keep my ambient temps up but I have seen so many people recommend providing belly heat for balls. Any advice would be great especially if someone has experience with those enclosures. Thanks very much.