Radiant heat panel, general heating questions

Hey everyone! I have a ball python in a terrarium with a screen top. In a few weeks, I'll be getting a bearded dragon, and the beardie's cage is going to go on top of the BP's (both are custom cages with front-opening doors). While there will be spacers between the two cages to allow ventilation for the BP, it won't be enough to keep using a heat lamp as I am now, and so I was planning on getting an RHP after hearing so many good things about them.

However, after a bit of research it doesn't appear that RHPs can be mounted inside a terrarium with a screen-lid. Is there in fact a way to safely do so?

If not, I'm happy to look at other options. I've tried the ZooMed heating pads in the past and had trouble getting them to stick, plus they didn't seem to offer enough heat on their own. The Ultratherm heating pads at ReptileBasics look a bit more promising and are what I was leaning towards if an RHP isn't an option. Has anyone had any experiences with those?

Basically, I'm just looking for a heating source that will go inside or under the cage, vs. on top. The cage itself is 14" high, 18" deep and 4' long; the room temperature is 73 to 75 degrees year round. Aspen is used as a substrate. Thanks!