A remix of:

https://www.thingiverse.com/thing:4189133

I added a dovetail mount that screws to the wall. The hanger then slides over that, hiding the screws. 

PRINT AS SHOWN IN THE .3MF FILE. It also has the infill, perimeters, and seam alignment settings you will want. Printing the the wrong orientation could lead to bad things for your instrument.