Of course a brick would have a ridiculous amount of drag but that was not my point and my example was extreme. A sheet of plywood will fly off the top of a car if it's not strapped down and that has not aerofoil shape.That seems wrong. The smooth flow of the above-wing air being drawn downward provides a large part of the lift force. I think that the turbulance behind a brick would disrupt that. But I have to admit that it is not something I have studied.