[Tubbytiger]

You could try wire mesh/chicken wire to cover the hole. You would make little holes into the cage side and cut out a piece of chicken wire suitable to cover the hole. Then (from the outside or inside, doesn't matter it is a personal choice) you would use zip ties/cable ties to secure it in place. You could also just use metal wire instead of the plastic ties to secure it in place. If you wanted to use plexiglass, you have to use an animal safe silicon and that is usually called some type of aquarium silicon that you would use on a fish tank.