Right after instantiating the boxes, add these two lines:
box1 layoutSpec offAxisEdgeWeight: #rowTop. box3 layoutSpec offAxisEdgeWeight: #rowBottom.
box2 is centered by default.
Figure C.1: Top to down
Edit the example as follow:
... spacer1 layoutSpec proportionalWidth: 1. ... spacer2 layoutSpec proportionalWidth: 3. ...