I can see a version of this that has a deep ‘U’ that has a narrow U closer to the wall, and expands to a larger U away from the wall. This allows it to work with the smaller neck of a ukulele or mandolin, and also the larger neck of a 1 3/4" Guitar.
An alternate design has stacked U’s, with the lower U is fixed and large enough for a guitar. A second smaller U for a ukulele or mandolin is on a hinge that folds up against the wall.
