carriers that i use to transport them to the vet. i also use a heavy-duty black wire pet crate that has room for both of them to ride in. i made a soft pillow for them to lay on in the crate. they don't seem to mind it too much. once we get to the vet, we immediately put them in their carriers to take them into the vet.
it is a bad idea to take your cat in the car without a carrier or box, i know from experience. because speedy is a pretty mellow cat, i thought she would not have much trouble riding in the car with just 