The STL included has a 55mm wide gap and is 75mm from the top of the mounting block to the beginning of the bump (so 85mm from the wall).  This is probably fine for most guitars, but you can mess with it as desired.If you change the dimensions, be aware that rendering in OpenSCAD is quite slow due to the Minkowski function used to round the edges (rendering can take 30 minutes or more even on a relatively fast machine).  Changing $fn=20 to a lower number will speed up the rendering (at 10 it's more like 5 minutes, but there are visible bumps in the curves).  You can comment out minkowski to render in a few seconds until you get the thing looking about right.