If I remember correctly, it doesn't take a lot of heat to melt your standard sheet. We worked with it once to set up an experiment and I believe we rounded the edges by placing it in an oven. What I would be more concerned about would be the heat transfer coefficient of that versus glass. The heating characteristics would be very different and you could get a very hot spot instead of an even heat dispersal pattern. In that sense there could be a greater risk to cracking too. You might even want to consider how much it expands under heat, you could be breaking your seals. It for sure wouldn't be as good as glass at withstanding repeated thermal cycles... meaning it will warp eventually, even if you don't exceed the melting temp.
I personally know people have good luck with bottoms of glass or wood and SIDES of plexi, but I'm concerned about bottoms of plexi if you have a UTH.
I guess after all I'm not much help, if I was at the office in 5 minutes I could set up the thermal model and tell you what the temps are and if it melted, but alas it is the weekend and thermal properties found on the internet are not to be trusted. 

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