There are quite a few forms of Lexan some are definitely overkill for what you need, Your UTH should give off significantly lower temperatures then the Flame rating of any form of Lexan, when we did some heavy bending on a sheet for a painting enclosure a year or 2 ago we used an Acetylene torch on low (still bloody hot!) to soften it for the forming and we cut it with a metal grade cutting zip-blade on an angle grinder so its pretty strong stuff. You might want to look at LEXAN® MR10 though as it is more resistant to scratching then the general purpose 9k series stuff we used. We cursed a few times on the job when something little in the task put scratches on the finished product, it would depend on how much traffic you'd expect to have around it I guess though and how big a deal it is, you can buy stuff to buff out the scratches too.
Hope that helps some. I'm planning to use some myself, but on a custom top enclosure I am designing for a vivarium, not a bottom, that will be made of an old 45 long aquarium I have.
Good luck and would love to see the design/finished product if you do go with the it.