I made the first prototype in a quick and dirty version.

I designed a second one that is currently being 3D printed but It won't be finished by the end of the contest... This one replace the plastic bag with a more optimised geometry. It will be watertight and filled with oil like the first prototype. Calcute the force in first configuration is quite easy but I don't have the mathematical knowledge to calculate it in the 3D printed one. (if someone have a few minutes to spend explaining me how to do It I would really appreciate It)