Have you considered polycarbonate (Lexan)? It's more expensive, but it is a little more scratch resistant, doesn't yellow like plexi and is virtually unbreakable.
My father works in the plastics industry and after speaking to him at length I'm going to use polycarbonate for several cages I'm about to build.

You can find it on the internet with a little searching or go to a local supplier of industrial plastics if you can find one in your area.
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