A (fish) tank could be good for her. Both of our dwarf hamsters live in 10-gallon tanks, but 5-gallon tanks are also good. You can buy screen tops that fit comfortably on top for ventalation, and I believe there is some kind of metal clippy-thing availible that can fasten the screen top on, so the cat cannot get the screen off. (Can't remember what it was called.. but we used to have them)

Good luck!