I knocked this rail together from some aluminium strips used to aid plastering the corners of walls:
It is about the width of a 1.5 inch tyre and long enough for the Twenty to be strapped into. The rail and the Twenty can then be strapped onto the Yuba Mundo for short trips to meet a bikeless person. The Mundo actually rides pretty well with this contraption on it. It would be feasible to have one on the other side too.