Thankfully, we live in the age of smart leds, because the WS2812B make the task of putting over a hundred led into a thin fretboard much easier. I cut an led stripe into its pieces and usesd the template on the left to arrange them and soldering them back together.

After the dirty work I printed the fretboard shape again using wood filament and a better quality and put the led-skelleton into the new shape. Unlike the templet-version, the led holes dont't go through the whole fretboard but have a one layer cover, making the leds invisible when off.