Been there.

The best place for a bike is inside the car if you can manage it. That's where I put mine (and I use a bungee to keep it there for safety purposes).

As for hanging the bike by the top tube:

1. Rather doubtful you'll do any structural damage ... even with a CF bike.

2. If you use it often, it's almost certain that you'll mar the finish wherever paint/clearcoat touches the rack. Consider the 3sigma situation ... windy drive in the desert with scattered rainshowers. No matter what you do, the wind is gonna buffet the bikes around, and any debris blown into the interface is gonna leave it's mark. The screen protector stuff may solve that problem, so long as the bike doesn't shift and it's properly positioned.

I guess it depends on how persnickety you are about your bike's finish. All of mine are old enough now that I really don't mind a few minor scratches. I've got a few of those myself, so why should the bike be any different?