The three speed shifter has two removable threaded round pieces, the "knob" and the "plunger". They are "female" threaded.
Each piece is removed with counterclockwise rotation.
Then drill your knob "grenade" on the bottom to fit the handle and on the top to fit the threads AND the "plunger". Then make a "plunger to fit the upper hole and also fit the inner threads.
If you have a drillpress, this should take 20 minutes with the proper drills and taps.
You can even use the drillpress as a lathe if you want to in order to shape things.
The pics of a shifter are below for you to picture the male threads I'm talking about.
View attachment 749778
[ Above picture swiped from Interweb posting]
[EDIT - You can consider drilling only the underside of the "grenade" and then tap it for the inner plunger only. Drill the thinner hole first and then measure. Drill the outer larger hole 3/8 inch shorter. Then tap the inner hole with a LONG tap, or just fight to twist it on. Don't overtighten. Then just push your entire "grenade" down to shift. Remove the limiter washer on the outer threaded tube to allow a smaller hole that won't allow the device to flex every time you shift. Fit a small metal washer on top of the smaller diameter inner threaded piece to prevent overtightening and wear.]