Purrley,
This might sound simplistic, but why don't you get a baby gate and block the doorway? Little Tess couldn't get through, but the cats can easily jump over it. It is no more than 30" high. You can get one at WalMart, KMart, etc. Mine has been a lifesaver, and although my big Golden Retrievers could just jump it is they tried, they have never tried, so the litter box and the cat food are safe.

Good luck.
Logan