I bought one (forgot the brand name) from Petsmart - the one that you hang on the car seat back. Kimchee doesn't like it since he cannot lay down, he has to constantly sit due to the size of the car seat. One time the snap has gotten loose and Kimchee got out of the car seat. I am not sure how it happened, may be he was too exicted to get out and he pulled too hard and the snap open.
I immediately returned it to Petsmart.
I now have a lookout car seat for Kimchee. He loves it because he can lay down totally or sit and look out whenever he wants. It is also very secure in any way.