I designed this shift knob for my Subaru, because I wanted something that was weighted (for 'easier' shifting); the standard shift knob was only 196g, and that simply wouldn't do. Thus, I found a cylinder of lead that was lying around and fitted it into the center of the design, which brought the weight up to a hefty 512g.
Although this design is highly specific, perhaps someone can use all or part of it to improve theirs.
(1x) M12x1.25 standard or Nyloc nut (for the actual shifter shaft)
(8x) M3x35 bolts (for attaching the two halves of the knob together)
(8x) M3 standard nuts (function as embedded nuts to keep the halves secure)
After installing it, my shifts have become slightly smoother and even after sitting in the sun all day it hasn't lost its rigidity.
UPDATE: (2017-08-14) Still shifting wonderfully and has survived a few 40C days in the sun; a testament to PETG's temperature tolerance...
I printed this in PETG for its strength and heat tolerance (since it sits in my car all day and becomes hot and PLA would've become too soft).
I printed the bottom half of the knob without supports and it managed to do it just fine; however, depending on your printer (and material) you may need to use them.
I designed the M3 nuts to be hot pressed into their cavities as well as the M12 nut; however, you may want to double check the size of your M12 nut when you buy it. When I first tried to hot press it in, it was simply too small, so I had to remeasure, print, and press it again.
Also, I used a 3mm drill bit to carefully clean up the holes of both halves, as PETG can be fairly "stringy".