When I bought my 1995 MX5 Mk1, there was an aftermarket, fake carbon fibre gear knob. The gearknob was about 100mm long which I felt was too long and detracted from the short, comfortable changes I expected from the box.
I designed this gear knob as a replacement. It fits snuggling in the palm of your hand and provides short, really sporty feeling gear changes. The gear knob screws directly on the existing thread and includes the chamfer for a tight fit. The length of the rod sticking out beyond the gear stick cover is 50mm with a thread 10mm x 1.25. This gear knob just covers that and is about 54mm tall.
I smoothed out the print by sticking a 10mm threaded rod into the knob and the other end of the threaded rod into a drill. I ran the drill at a medium speed. I used a soft sponge soaked in acetone and then used it to get rid of all the printing defects. This gave me a finish comparable with a snooker ball. Surprisingly this only took about 10 minutes. Don't let the sponge dry out or it gets sticky quick.
I finished the MX5 gear knob using a high gloss spray paint. 5 Coates and around 16hrs drying time.