I have a feeling no magnets will be necessary! Great Job Slanks, can't wait to see it in action!That is inspiring!
May I suggest ... Hold down the lid and ghoul with weak magnets at the opposite end from the hinges. Add a stout spring to the end of the air ram. Air pressure will build up in the mechanism stretching the spring until it overrides the magnetic grip, and the lid will pop open suddenly, even though the air cylinder is extending more slowly.