I'm of the opinion that the simplest solution is usually the best.
I am therefore sometimes confounded by the contraptions made to carry a 20lbs bike on a car.
I have something similar to this... two of them, actually. One I have mounted permanently to a roof rack & the other is mounted to a removable roof rack that I can put on any car.
http://gaby.fachrul.com/img/mountain...-kb-jpeg-x.jpg