I printed 0.16 layers in clear PETG. I cut two pieces of 3M 300LSE tape and pressed the button firmly into place.

For whatever reason the LED is not centered on the Tonal I used to design this. So the add-on button is direction left and right as well as to match the curve of the bar. I made a centered version in case they aren't all this way.

(Note that the Tonal bar button is rounded/convex, and this button is made to mirror that. So if you flip the model it won't print accurately.)