This spring I had the opportunity to learn by accident that, a piece of plastic wrap (from the kitchen) works as a good cover for melting wax. I had some cut comb that I set aside during extraction, and didn't want bees of flies to get in it. So I covered the container with plastic wrap. 45 mins later, I saw the comb was mostly melted. I wasn't happy about it - but it was educational.
So I'd guess the plexiglass wouldn't have to be very thick.