I've gotten Rizzo the outward hound car booster seat. It's super easy to put on and take off, and since having him ride in it he isn't quite so bad about car rides. He doesn't run and hide when I grab the keys and call him, and I've even gotten him to hop in the car
though, he still gets a little shakey, but it's not as bad anymore. Apparently being able to see out the windows is calming. Good luck, I got the booster seat from PetSmart for $25 and it's really sturdy.