There are a few things you can do. I recommend going with light diffuser. You can get it at Ace or sometimes Lowes or HD.
It is pretty cheap and easy. You just cut it to fit so that it fits in the grooves up top. Then you can also cover it with mesh so the holes are not big enough for anything to jump.
This will allow for better gas exchange and less heat, where as glass would hold everything in.
I believe that most of the people here (all that I know in real life) use the light diffuser. (sometimes people call it egg crate too).