Summary

I used the awesome d20 model by whosawhatsis to build this shift knob that is now installed on my 2014 Jeep Wrangler.

I've included the original d20 model and my OpenSCAD source code so you may adapt this to your vehicle. I've also included some photos that show how I measured my shaft.

I recommend flipping the model 180 degrees for printing which works mostly well without supports.

Update Nov 1st 2017
I've added a rounded corners version along with a variable to change how much the corners are cut down.