I wouldn't do it. It would be very prone to cracking and you don't really want that in a gearbox.
There may not be traditional "impacts" but there would be stress placed on the plates. Although, if laser cut, plexi might not crack as much.
Last year, we put a piece of plexi between two 2x4's and used 4 bolts to hold it on. One of the bolts was slightly misaligned and it managed to create a huge crack in the plexi when we put it in.
I'll stop ranting and let someone with a bit more experience answer your question.
-Vivek