This is an updated version of my previous volleyball-shaped shift knob.  I found the original came loose over time, so I modified it to contain a cavity to insert a nut that threads onto the shift knob.  The cavity is a 17-mm-wide hexagon with a depth of 10mm.  Use your slicer to insert a pause in the gcode at 32mm and insert the nut that fits your shift knob.  Then continue the print to encapsulate the nut.  This model attaches very tightly and has not let go for years, but it's easy to remove if needed.  

Note: the STL file contains one insert and one main body.  Print these as separate parts if you have a single-filament printer and want to create a multi-color print.