Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5162145.

A beef-supreme redesign of the traditional Technic ball 'n socket joint, for when your Bionicle MOC is too heavy for its own knees to support.

EDIT: Updated to make it easier to pop the joint into the socket, prevent it popping out as easily, and add more friction.

I highly suggest printing this with a Brim to keep the corners from lifting and deforming. Also needs a support block on part of the socket piece.