[Gunner]

I was considering grinding down the jam nut, but I'd prefer a smooth or knurled outer edge over a hex.

I might try the washer thing again. We have a hardware store here that has a tremendous fasteners section, so finding a thick (.010) chrome washer was no problem. I couldn't come up with a good way to clamp it without damaging the finish, so it slipped a couple of times and the tapping was flawed. I think I have a way to hold it - clamp the sh*t out of it in a vise with hardwood or poly blocks - for drilling and a clean tap. That would give me a nice, thin, visually appealing lock nut.

I might also start with a knurled lamp nut of the right size.