I have a hatchback, so my back seats are always down and I use a wire crate, with bed and blankets for both my girls. My schnauzer is more comfortable in that then in anything/or with anything else in the car...and it stops Charlee from jumping out, falling off the car seat I had for her (2X it happened)...once she was hanging outside the car seat by her harness
I feel so much better with them in a crate! And yet, they can get lots of air, and see out!