The plexiglass is just held on with velcro. It works quite well, I've never had a piece come off unless I was pulling it off. I also started painting the outside rim of the plexiglass, so that you can't see the white velcro underneath, and it looks better. I haven't finished painting them all yet though.
Semi-transparent covers would be super sweet! Especially if they were solid and smooth. If you figure out how to do that, definitely let me know