Pet kennel. I use one like this for long-distance travel with my cats. It fits nicely in the back seat and one side can be used for sleeping and the other side for a little high-sided litterbox and a little food. You can put a little water on that side as well, but you need a container that can tolerate sloshing!
Dog Houses 108884: Large Dog House With Porch And Heater Pet Kennel Deluxe Rustic Wooden Heated -> BUY IT NOW ONLY: $338.98 on eBay!