I don't have a cat, so I'm not sure what else would work other than trying to catproof a cage using the organizational shelving. Perhaps you might want to visit Guinea Lynx or CavyMadness and see what they have to say about it. I'm sure there's a number of people who have dogs, cats and guinea pigs in any sort of combination, they might be able to suggest ways to make a good habitat.

Guinea Lynx

CavyMadness