Long ago, Mary picked out a PTT switch with a raised, square post that provided a distinct shape and positive tactile feedback:
Time passes, she dinged her thumb in the garden, and asked for a more rounded button. I have some switches with rounded caps, but replacing the existing switch looked a lot like work, sooooo:
As with all small objects, building them four at a time gives the plastic in each one time to cool before slapping the next layer on top:
The hole in the cap is 0.2 mm oversize, which results in a snug press fit on the small ridges barely visible around the post in the first image:
Rather than compute the chord covering the surface, I just resized a sphere to twice the desired dome height (picked as 6 threads, just for convenience) and plunked it atop a cylinder. Remember to expand the sphere diameter by 1/cos(180/sides) to make it match the cylinder and force both to have the same number of sides.
If it falls off, I have three backups.
The OpenSCAD source code as a GitHub Gist: