Use Lexan or glass... plexiglass tends to discolor and craze (tiny spiderweb of cracks) under heat and light. Lexan or glass won't do either... Glass is cheeper and easier to work with but can crack if you drop it... lexan you can hit with a hammer and it doesn't matter.
I think glass is easier in the long run to keep clean and has a lower refractive index than lexan.