This may have already been done and I missed it, if so, please disregard.

I have a stock 914 side shifter and a Numeric Racking shift knob I wanted to bring together but the Numeric is a bolt on knob, not like the stock mounting, here's how I put them together:
-found some 7/16" steel rod in my garage scrap metal pile and ran a 7/16" drill a couple inches into shifter handle...The ID of the handle is something like 0.008" under 7/16" (.4375"), so it's easy....
Click to view attachment
-Using my crappy small lathe, pilot drilled 7/16" rod and then tapped M5-.80 and checked fit to drilled out shifter handle....
Click to view attachment
-slid 7/16" rod (insert) into shifter handle with some Loctite to just below flush with the shifter handle end and then using existing 4mm hole in my shifter handle, drilled through insert and hammered in an M5 spring roll pin putting the pin into double shear, captivating the insert and ground off the excess of the pin sticking out the other side....
Click to view attachment
-Then I just had to screw on the shifter knob mount which also hides the role pin and then screwed the knob on, easy peasy...
Click to view attachment

Hope someone finds this useful...

Nicely done and great alternative to a stock knob too.

Sharing your fabrication work is always a great thing - you never know who will be inspired by what you've done.
And that was done nicely, too. Thanks for sharing!

I might have to buy one of those....that might be the coolest shift knob I've seen. Maybe it's the simplicity of it.