Additional Rear Access door/cat door

I'm thinking about installing an additional access panel door on the back of my 3124Ks. The purpose is to have a cat door I can install in the panel when I take my cat with me. The swinging cat door would be smaller than the access panel, then I plan to hang a cat/dog travel cage so the cat could enter the travel cage to do her business in the litter box, thereby keeping the litter box outside but protected from the elements under the bed. Maybe hanging the cage with something like what they use for grills.

Any suggestions, advice, warnings?

Thanks

Hope there are no racoons around when kitty needs to use the litter.