Well glass tanks are notoriously hard to get set up right. First i would cover 3/4 of the top of that screen with something like acrylic or even cardboard wrapped in tin foil to help hold in heat and humidity. Next i would cover the sides of the cage with black craft paper.

If you cover the top, your heating issues should be ok. I have no idea how cold your room gets at night or stuff like that but you can also maybe add a second smaller CHE on the opposite end to just heat the air. That combined with covering 3/4 of the top should take care of your problem.

I have seen guys use RHPs on Exo Terra tanks for GTPs. You need to highly customize it though. You would have to ditch the whole screen lid and then have a piece of acrylic cut to fit the top, then drill holes to mount the RHP and then silicone the whole acrylic top to the frame of the cage. I dont remember the name of the video but i'm sure you could find it on youtube. It was done really nicely but for the amount of work it took, better off just getting a PVC cage imo lol.

Bottom line is start by limiting the airflow as you are losing most if not all of your heat through the screen top.