Blocking screens on top of enclosures...what is the best option? Acrylic, PVC sheet?

[Malum Argenteum]

Ultimately, you should consider a proper PVC cage IF you are having trouble keeping heat and/or humidity in. These tanks can use Radiant Heat Panels (RHP) and heat tape, etc. and do a great job of keeping both heat and humidity in. They can also be much bigger and much lighter (relative).

This, really.

I use acrylic over the screen tops on ExoTerras used as dart frog enclosures (no supplemental heat), and other than the moisture induced warping that wouldn't be much of a factor in a BP enclosure it works fine. I've built a few things for aquarium use out of heat-bent acrylic; it does not take much heat to soften it, and I wouldn't get it anywhere near a CHE. No idea what safe clearances would be, though.

Something better looking than aluminum foil might be aluminum sheet. Tools would be needed to shape it (a hole saw to cut out for the CHE and then a file to smooth the edges) but it would be safe and likely look pretty decent.