Hello Sidius! I like what you did with the polycarbonate sheets! The gap around light fixtures is also good for providing some ventilation. Next time I have a whole tank top to deal with will try your idea.

I'm using saran wrap and the trick is to affix with tape to top edges. That way it goes with top when you remove it for maintenance or feeding. For example on an Exo Terra top you can actually seal the whole front half with saran and tape around edges, leaving the half back for light fixture. One of mine did curl up a bit around a lamp fixture once due to heat; but it was minimal and did not looked burned. Good luck .