Fran…another good thing about a crate in the car is that if they get sick, the cleanup is a lot easier.
I use bungy cords to secure the crates so they don't move. In my last car I would hook the cords to the bottom of the seats - in my new car there are actual lash points. I also put bedding in the crates that doesn't slide around - along with a stuffed toy and a dental dinosaur.
With a sedan you should be able to find a crate that fits in the back seat (check out a wire 200 series or vari kennel medium size - most likely with a soft crate they will chew through it). My new car can fit 2 crates in the back seat without folding the seats down. If you have a coupe, front seat would probably be your option.