It's not only heat you're keeping in by covering much of the screen top, but for BPs (& some other species) it's also the humidity.
Both plexi (expensive!) & foil, also many other materials, will equally prevent air-flow. It's your choice- one isn't 'better than the other'- mostly a matter of preference, including appearance.
You might prefer foil, because it's lighter weight, for example. Try it & go from there if you're not happy with it. (Foil also tears easily though.)
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