I have this one:
Pet Gear Medium Booster Car Seat (Interior Measures 10" x 14" ) | Overstock.com and like it alot. Mine like it so much, they are always using it for a bed when it is in the house. I wish it was just a little deeper as I would feel safer if I had to hit the brakes hard. But I have not had any such problem with it. They seem to stay put nicely and enjoy getting to look out the window. Annie, especially LOVES to ride along.