An enclosure would be the best answer. I know you can also get cat fencing that goes around the top of the fence and is *supposed* to keep cats in. Ally does get out occasionally on a harness and leash, but once managed to wriggle out of it, and we had a terrible time getting her in again.

My recommendation would be "Don't tempt fate". If Cubby is a happy indoor cat, why rock the boat?