I modeled this shifter knob after the same shape used by the ProSim sequential shifter knob. My thought process being that a high-end piece of hardware like that most likely comes with decent ergonomic. I would also recommend printing this one to a thick density and sand it to a very smooth finish.

One of the two versions of the knob has an additional cavity at the bottom, so that it is also compatible to this mod,

https://www.thingiverse.com/thing:4745214/files