This is a shifter knob design I made to screw onto an 8mm threaded rod (you'll need to insert an M8 nut during printing). You can adapt the model to fit other types of fixations (the STEP files are available for download.)

I personally stitched fake leather onto it, but that’s optional. I designed the model in two parts because I thought it looked cool and also needed something to hide the bottom edge of the leather. The larger hole at the bottom of the shifter knob is meant to hold a second M8 nut, which helps ensure a very secure attachment.

To fasten the bottom part to the top, I used three screws with a 3mm body, 12mm length, and 5mm head. It's not the optimal size (I just used what I had on hand) so I recommend modifying the model to use smaller screws if you have them.