This is what I'm using for now. It's a strap that goes inside your car. It holds pretty strong. It's like a Thule roof rack system but with suction cups on the foot of the rack. The straps goes into your car. At freeway speed, you will hear wind noise. Not recommended to use during rain as the straps can get wet and caused the water to get in. I've carried 2 bikes on top of the car at 75 - 80 mph with no problems.
I didn't want to do a trunk mount due to worrying about the scratches if the trunk rack moves around too much.