I print things occasionally, and sometimes can't remember what size nozzle is on the printer. I couldn't find a design, so created one in about 20min on Fusion.
My indicators match the nozzle indicators for E3D V6 nozzles (zero dots for 0.25mm, 1 dot for 0.3mm, 2 dots for 0.35mm, 3 dots for 0.4mm, 4 dots for 0.6mm, 5 dots for 0.8mm, and 6 dots for 0.5mm).
This is attached to my 2020 frame as seen in the picture. The moveable indicator simply sits on top and uses the 2020 track on the TOP of the frame to hold it in place. Very simple, but efficient.
The one problem is I can see that the moveable indicator could easily be knocked off by accident by my hand/arm occasionally.