Please remember that anything not properly secured down in a car can become a projectile in the event of a crash, especially a rollover. The carriers that have a strap to go over the seat back are not secure. The following is a quote from an owner's manual for a targus carrying case:
"Any person who uses the Targus Portable DVD Vehicle Travel Case assumes the risks, including bodily injury, casue by the presence of the Targus Portable DVD Vehicle Travel Case product in the vehcile during an accident involving the vehicle. This product has not been used in any crash tests and its presence in a vehicle may become a dangerous projectile during an accident or other sudden force upon the vehicle."
I realize the decision to use such an item in the car is a personal one, I just wanted to add that so you can make an informed decision as well.