Inlet a plate and magnet on at least 2 sides of the hatch (all 4 sides if the hatch is large/and or the plane is going to be flown at high speeds).

The edge of your fingernail is normally all that you will need to open the hatch, yet a 2 magnet system on a hatch 1"x2" will stay inplace at 60-70 mph with no problems.