If you are thinking of using a cardboard box for cat transportation it had better be a very strong one, as I found out many years ago when I took tabby on a 5 mile bus ride to the town centre vets in a standard weight cardboard box in which I kindly made small holes thinking it would be nice for kitty to peer out through. First came claws, then a paw then a whole thrashing leg, followed by an enraged, biting head. There was not much left of the box by the time we somehow made it to the vets. She came home in a proper cat container.