You may want to put tape around the sharp lip which remains inside the can. The depression at the bottom of the can helps keep the force centered over the plunger. Removing most of the needles with metal snippers is possible if clay is used as the closing mechanism. This will minimize the chance of a puncture injury resulting from an accidental stabbing with the needle. Although mass is plotted, the actual units should be units of pressure.
Pressure is force divided by area.