A Control Line stunt flyer some years back rigged an airfoil testing setup for the hood, not the roof of his vehicle. The roof sees turbulated air and it's most often upslope air. In fact, unless the airfoil is significantly forward of the nose of the car it's going to see disturbed air too, just less disturbance.

But since he was looking for comparisons more than clean, pure data, it worked for him. And he set the competition environment on it's ear with the things he did thanks to that testing.