I'm sorry, but I don't think the ultratherms are going to do it. The ultratherms will make for a nice basking spot, but they won't help with your ambient temp. Assuming the ambient room temp is less than 80F, you're going to need additional heating.
That's where the screen top becomes an issue. Screen tops don't retain either heat or humidity. If you add a heat lamp, it may raise the ambient, but most of the heat you're pumping in there will escape immediately and it'll take your humidity with it. You can cover the screen, but it would be better to just replace it.
Zen's site mentions that the panels are replaceable. I just don't know if that includes the screen top. I suggest that you contact Zen and see if you can replace the screen top with a PVC panel. If you can replace the top, then adding a radiant heat panel (RHP) is what I would advise for raising the ambient. RHPs don't get hot enough to pose a fire hazard and they don't emit light, so you can leave it on 24/7. They, also, don't dry out your enclosure the way a heat lamp or ceramic heat emitter (CHE) would. A deep heat projector (DHP) is another option to consider.
If you can't replace the top with a PVC panel, you can still partially cover it (aluminum foil tape), and add a DHP or CHE. DHPs and CHEs are preferable to a heat lamps because you'll need to keep the heat on 24/7 and the light from a heat lamp will mess with your BP's circadian rhythm. DHPs and CHEs don't emit light.
Sorry. Probably not what you wanted to hear. You should still try using just the ultratherms just to see if it'll be enough. I just don't think it will be.










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