The "BMW" knob had released itself from its threaded insert so unless you had a gentle touch the knob would come off in your hand while rowing through the gears. While this can be amusing, especially for passengers, it can be a bit distracting.
I've had a dense plastic ball sitting around, much like a cueball, but slightly smaller. Around 1.75"
I drilled into it and used a dremel to hog out the hole to the same size as the insert. A little JB Weld later, a nice shift knob.
It's not too large, its s bit weighty which tends to help with shift feel, and the plastic will stay relatively cool regardless of the summer temps.
(Its not radioactive)
I had an aluminum shift knob in my Miata and had to keep a cloth in the car for summer driving.
Those seats have got to go!