This was what I used for Lolly. I started with her in the front and then slowly moved her to the back seat and then the boot as her confidence grew. I used to put the seat belt through the handle at the top and plug the seat belt in to keep her and us safe. This is where I got it
But I'm lucky that I am near enough to visit this pet store. I can't remember what size I got. I think it was the medium one going by the price.