Here is the second ( and final) version of the printed insert. I fixed the capping of the end, and adjusted the 1 mm gap.

Here I checked the fit in the knob. very nice fit.

Testing the fit on the shaft of the switch. It fit well, though It is loose enough that it can be removed without too much force. I allowed for 0.2 mm of clearance from the actual measurement of the shaft, but I probably could have gone with 0.15 mm.

Final placement after gluing the insert into place. I used some UV curing resin to tack it into place quickly, followed by superglue. I probably could get away with just the super glue, but I had the resin on hand.