Plexi and acrylic will warp. If it is small enough, you can keep it from warping with ridges, I have an acrylic cover on a 2.7 gallon tank with a ridge front and back that stays flat, but it requires fabrication.
Mesh will work if you just want to prevent jumping. It allows a lot of evaporation, which can be good or bad depending on your system - helps with cooling, hurts with stability without an ATO. Glass will prevent evaporation and jumping. Light transmission isn’t a worry, both glass and plastic are about 96% efficient in transmitting light, which is probably as good or better than a screen.
glass works, I would spend the extra money to have a glass shop cut it and smooth the edges rather than getting a window pane from HD are a hardware store and always worrying about cutting yourself.
