I wanted the ankle to allow rotation and bending. This was a challenge but I had a thought about how to accomplish it. By layering cardboard with a central point of rotation, I can build a strong hinge joint that will still allow the bend. By building that hinge into a rounded base, I can allow it to rotate. At least that's the plan.
So the first thing was to design the ankle to allow it to bend forward and backward. That is where this piece comes into play: