Okay, so I’ve come to terms with the fact that there are lots of different fret designs too (not sure why I was surprised by that).
I have settled on a design that

• keeps the full, 15 mm fret height
• four different ‘insets’ of the bottom (1.5, 2.0, 2.5 and 3.0 mm)
• mostly hollow inside (there are stiffening ribs) to keep weight down
• solid, flat bottom

I have also re-packaged all the zip files to include the guitar type, game system and version number, Additionally, the zip file includes

• a TESTER fret
  that has no side or end guides – to use as a gauge to determine what ‘inset’ you will need to print.
• a drawing with 5 key dimensions
  to check against your OEM buttons before printing and to verify the final printed button size is accurate.

All designs are now based on a fully parameter-driven design, so the design is consistent across all guitars/systems (the dimensions, of course, vary per guitar/system)

A new layout for the STL files is provided as a much easier to read/understand matrix (image above).

STL files are still free (no guarantee or warranty).

Finally, a more professional feedback form is added so you can tell me of your experience with the buttons.

Now that I’ve got the basic house-keeping in order, I’ll focus more time on filling in the blanks!