Tom
Make a card board template of the opening and go see a glass shop to have one cut. They will need to allow clearance for the gasket, which should be available from most auto parts places or the dealer. I know bobcat sell this gasket by the foot.
It is a 2 piece gasket that has a strip of rubber forced into a groove to secure the window. Make sure the gasket is this style, becuase the operator needs to be able to pull that "locking strip" out so he can remove the rear window to escape in a emergency.
Ken