I use a Thule rear rack for my mountain bike on my current car, it has movable arms and adjustable straps. So I will just readjust it to the Sonic...
Coming home from a ride exhausted, it's too easy to forget the car on the top rack and drive into the garage
What Thule rear rack are you using on your Sonic? I have the hatchback and definitely agree with your roof rack comment. Plus, I don't want to decrease the gas mileage.