I've used polyurethane (3-4 coats) in the past, but I'm planning to use fiber-glass in the future. Plex will work too if you silicone the corners well.
Heating in my house is done with minimal effort usually incandescant lights and/or flourescents. I use heat pads inside some enclosures, but make sure they aren't too hot (protective pad underneath).
As for glass, when I'm making something that is furniture quality, I always prefer 1/4" tempered glass bought from a glass shop. Measure perfectly becuase tempered can't be cut after it's been "tempered" (a heating process, I believe). Tempered glass looks better, but it runs up the price quite a bit. If you don't use tempered get double strength plate (not from Home Depot).
Good luck,
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Zoodude
"Just because I'm paranoid, it doesn't mean they aren't after me."