We’ve always used purpose designed dog harnesses attaches to the rear seat belt points. After a very bad crash in our Puma Black - a giggling teenage girl (lucky for her I wasn’t there) hit our stationary car at 60mph+, no attempt to brake, sending our car into the one in front and that into another. When my wife thought of what would have happened had our two dogs been on the back seat.....wouldn’t let any of our dogs travel unattached (I know you’re not suggesting you would either, but for anyone else considering it....).