Maybe you can get some ideas from the pictures. On my last rudder I had a coiled bungee like a spring that was located inside the alum. housing that worked holding the rudder down but was hard to install. On my big present rudder I am building, I think I will try a bungee setup like the one in the top picture. Looks simple and easy.

I am building what should be the ultimate leeboard and rudder for efficient sailing. The leeboard is the NACA 0009 foil shape and for the sail area that I may be using I will have 36 inches of leeboard underwater. I know that's big and awkward. The rudder will have 18 inches underwater and is 12 inches wide. That’s pretty big also. Roughly 2 inches of the rudder should go forward of the rudder pivot for better balance as I understand it.

John