Building the diffuser was a little science in itself.
At first I printed the entire diffuser out of white PLA. Unfortunately, this meant that the neighboring segments of the lit segment were also at least partially illuminated.
The solution was to print the front of the diffuser from white PLA, pause the 3D printer after the first two layers, replace the filament with black one and continue printing. So the front of the diffusor became white and can be easily illuminated while the black frame parts separates the individual segments well from one another.
Actually the perfect use for a 3D printer with dual extruder. But it is also possible with a single extruder, just a little more laborious. :)