I did it for a friend who could not find spares anymore. He gave me the brown one.
I did the gray ones, with exactly same design as training exercise using BlockScad
1.- Draw a sketch, with empty desired delimite arrows
2.- To get the right measures by using a Calliper (calibrator)
3.- Try generate the object by using blockScad free web aplication.
4.- Save the .stl file
5.- Use Repetier software and your favourite slicer.
5.- Print it directly or by a micro SD card