I bought a pvc pipe with an inner diameter that almost matches the outer diameter of the shifter shaft. I did some hole reaming with a drill, this was the most time consuming part. Also depending on the inner diameter of your knob, you may need to sand the pipe a little on the outside so the knob fits. The shifter shaft has two opposite grooves open at the top. I used those as guides that keep the adapter from rotating about the shaft. Drill two holes in the pipe where the pipe interfaces with the grooves. Insert small headless hex screws in the holes -- they will guide the adapter along the grooves and keep it from rotating.Oh good I was starting to worry I was "special". So embarrassing when you have a passenger looool..
Still interested in how you made your adaptor.. I wouldn't mind a DIY knob!
I think you've explained it on other threads but i'm still hopeless!
Now for the trigger mechanism. The shaft has a plastic rod with a cutout, kind of like a huge needle eye. Make a cutout in the pvc pipe on opposite sides, get a metal plate that is the same width as the length of the cutout in the plastic rod (the closer the dimensions, the less slop there will be). This plate will act as the actuator for the shifter lock (just replaces the trigger). That's pretty much it. Now just need to examine how the know is meant to be attached and tweak the pipe so you can slap the knob on.
The pipe had a large enough outer diameter that I was able to use the ring at the top of the boot and it's nice and snug.
It may seem complicated "on paper", but is actually very straightforward. The most expensive part was the Momo knob.