<<<RE_UPDATED>>>I needed a stemmed knob for a project with a longer stem than I had already, so I designed one for the length bolt I needed and decided to make it parametric so I could use the file for other size bolts.  I used FreeCAD to design the knob and used a spreadsheet with a configuration table so that the size of the bolt could easily be selected.  The PDF file has an explanation of how that is done.  The file can be modified to add other bolt sizes and to specify the nozzle size, layer height, and tolerance you want to use.  The knobs can be printed without supports because the socket for the hex bolt head has layers to help prevent sagging.  I printed the knobs on my MK3S using VoxelPLA black with 0.2 layer heights.

<<<I made some minor changes to the spreadsheet values to improve printing and updated all the files for a better fit.  I also included a TEST object that can be used to test the fit of your bolts.  You still have to select the bolt size under Knob so that the TEST object can use the data in the spreadsheet.>>>