I would do some tuft testing on the car as it is now and then remove the trunk lid, make a cardboard mockup of your idea, and then do the tuft testing over again.
If you have a small webcam you could mount it on the roof of the car ( out of the way of the airflow of course ) and then just record it to your laptop which you would have in the car at the time.
Post you results here, since we all love to see tuft testing.
