You could get a piece of plexiglass and cut it to fit just right in the tank. Then you could get silicone sealant (like caulk....or use caulk....clear would just look nicer) and seal up any gaps, and hold it in place.
I have never done that, but to me it seems like the most logical choice. Real glass would probably work too, it just seems like plexi would be easier to work with.
I would just make sure the top edge is sorta smooth (use a file or something) and make sure it comes all the way up to the top so a little guy doesn't squeeze over and visit his neighbor.