I have this in my car, except I keep the legs folded in because they are really not sturdy.
Top Paw™ Booster Seat - Sale Category - PetSmart
DBF has this in his car, which she does really well with.
PawRider Pet Seat Belt Adapter
Roxie gets a little nervous in the car, and I think putting her in a crate/carrier would make her flip out. It took us over a year to get her to sit in the car without barking her brains out for the entire ride. Her previous owners used to let her ride in their laps, so she wasn't used to having to be restrained in the car.