I would recommend Pond Shield also. Can be a bit awkward to work with but when done right it looks great.
I use either mats or bulbs in all my custom builds. For a BP, I would go 18" high with a mat or 24" high with a bulb (and use a guard). I don't use the Radiant Heat Panels in my enclosures so I can't really help you with that one but going from previous comments, sounds like they work well. Might look into it myself in the future.
I wrote up how I go about building my custom enclosures. You can check them out here:
https://www.reptilerange.com/how-to-...ile-enclosure/
Let me know if you have any more questions and I will try to help.
Liam.