Seems like ypu have a couple options. Retrofit (safely) a board across the top half ypu will affix your RHP to and secure your screws to it...absolutely be sure to use a high quality thermostat, perhaps dimming or pulse, etc, do your research on what's best for your needs.
Or, you could mount it to the back PVC panel using said screws. Only thing is I'm not a BP guy so their husbandry is not in my wheelhouse...whether or not side heat will work may be best suited for BP people. It may not be illogical to have it sit on the screen top, but many factors like screen durability and thickness will make that call and it's not the best one IMO. Just don't sacrifice safety over convenience, and make sure your panel isn't too big, diminishing the gradient snakes prefer. Hope this helps.
Sent from my SM-G973U1 using Tapatalk