I have a sport with added Nismo wing. I tried pennies, nickels and removed the weight in the hatch mentioned above but still had the problem.
Then I bought a 6 pack of 1/4" thick 1" diameter circular magnets at Home Depot (Lowes has them also) for $2.
Stuck one on each side of the body in the rear opening right where the rubber boot on the hatch rests; the magnets are strong and don't move. Problem solved!
