My brother and I just made a custom lid for an exo terra for this purpose. For a RHP to work effecively, it must be installed inside the cage. They do not work well sitting on top of screen or through porous tops.

If you wanted to go cheap, buy a 1/4" piece of polypropylene and attach the RHP to the underside. You can then use a soldering iron to melt holes through the areas not covered by the heat panel.

To attach an RHP inside any cage, there are usually two screw spots where you screen it into the top of the cage.

Also, in my opinion, if you are going to upgrade a ball python to a "final" PVC enclosure from AP: Go for the T10. They are only $20 more than a T3 right now and you gain an extra 12" in length and 3" in height. While ball pythons do not really use height, that extra 3" is pretty handle from ease of cleaning cages and giving you some extra room for the RHP. The extra length is pretty nice for assistance is making a heat gradient and if your BP grows larger (say 2000+ grams), most of that 3' will be taken up by hides large enough for your BP.