I am also building my own device for squeezing my testicles. First version in wood. It is consisting of three sticks with circular crosssection each with a diameter of 25mm. Viewed from the side on the crosssections they build a triangle with even sides with the balls trapped in between.
Like holding you balls between two fingers and the thumb...
I planned wing srews for easy an fast handling. Additinally I try springs applying constant pressure.
Maybe I post a picture if it is finished, unless I am so happy with it that I prefer to sell it.
I also though of a construction of an alu version, but it is a bit more difficult to manufacture at home.