A company called Raceseng makes a nice cnc aluminum adapter. Its expensive though and is sort of designed to be used with their knob. I used it to install a wood knob my father made for me years ago. Works great if you can get past the price. I just tapped my wood knob to match the threads of the adapter. Pic from their website below.
There's also an adapter from 928 motorsport. Its not for a 986, but the concept is the same. As mentioned previously in this thread, its a slotted peice of bar stock. Shift knob uses a set screw.
I have also 3D printed ABS inserts for wood shift knobs that were used on a couple different vehicles. Something similar to these porsche adapters could be modeled and 3D printed fairly easily. It just depends on the knob you want to install (does it thread, use a set screw, etc).
