It's also worth considering to do what Minn and deadeye suggest only with a twist.

Hinge on one side and magnets on the other.

Airflow over fuselages often create areas of negative pressure and areas of positive pressure. Darn if it's a puzzle most times predicting where the pressures are going to help hold the hatch down and where they're going to try a pull them open. Put the hing on one side and since very few airplanes experience sideways airflow on the fuselage, the magnet on the other side probably won't see just negative or positive. (Unless the attitude in flight gets REALLY bad, and then you won't care if the hatch is found at the crash site or some distance away.)