You can't 'cat proof' just one hedge!

You will have to cat proof the whole garden. It will be expensive. There are possibly 3 systems:

1. secura-a-cat SECUR-A-CAT GARDEN CAT-PROOFING KIT

2. Cat Fence - Purrfect Cat Fence and Cat Fencing

3. Katzecure – cat fencing for cat breeders and cat owners alike