Things didn’t exactly go as planned. My original plan was to thread a bunch of nuts onto some threaded rod, and get them nicely lined up, then epoxy glue them together, and insert the whole thing inside the spot I had carved for it in the bottom. Because the shifter only has enough space for 6 or so nuts, but I wanted it to be really solid, I tried drilling out the interior of a few other nuts to lengthen the whole assembly, but still give room for the unthreaded portion of the shifter. Unfortunately my drill bits were not as tough as the nuts, so I was only able to do that to two nuts before my drill bit gave out. Then, my plan of gluing all the nuts together, waiting for that to harden, then put that whole assembly in the knob didn’t work either. The epoxy was brittle, and some of the nuts still popped apart from each other. I ended up using another adhesive I like (E6000 - it’s clear, and comes in a small tube). It seems to have done the trick.
I think altogether it was something like 17 coats of wipe-on-poly. I wanted to make it fairly durable.