Tried some chamfers to reduce the chafing & made a coffin shaped panel.  This worked well in 24 miles of running. The trick with freecad is only the part designer chamfer tool allows a variable angle & handles a combination of curves & lines.  The angle needs to be 57 to compensate for horizontal expansion.

Also tried melting the TPU button to make it fit tighter than can be achieved by 3D printing tolerances.  The mane variable is the horizontal expansion of the 1st layer.

Organizing the cord locks & matching them to each piece of clothing when they can't be washed is the new problem.

The sandal cord lock evolved to a perfectly symmetrical design with a triangle button.  All the laces have to be the same size.  The triangle might keep it more aligned than a rectangle.  Sandal cord locks have proven to eventually come loose over 24 miles.