I used PVC tubing.
Took the stock shifter off, measured the shift lever shaft diameter to know what diameter tubing to buy. The tubing I got was a little narrower, so I drilled it out a little (that was the most time consuming part). Then I arranged the tube, which I cut to the approximate length I would need, with the knob on it and put it on the lever shaft so I could mark the locations for the knob retaining screws. Then made the holes in the tube for the said screws. Also made a hole for the trigger pin (I half assed this part a bit). That's about it. I also made the knob retaining screws serve a dual purpose -- fix the knob on the tube and use the screws as guides on the shaft grooves so that the tube-knob assembly wouldn't rotate on the shaft. I used the stock boot without any modifications -- the top plastic ring fits very snugly around the PVC tube.
If it sounds complicated, trust me -- it's not. It was a fun little project and I'm super happy with the result. The total cost came out to about $40, with the knob being the most expensive part.
There's minimal play in the entire assembly, I bet no more than in BFI knobs. It doesn't rattle (the adaptor is nice and snug on the shaft, but with enough clearance for easy pull ups), and I drive over crappy roads around here, the whole car rattles.