In the first version I wasn't very happy with the clearness of the pixels. The matrix isn't flat, because the front plate has thin horizontal ridges the lat the light flow to neighbor pixels.

Dominic Buchstaller gave me a hint:
You can loosen all the screws of the panels so that the front plate can be taken away. 

Then you have two options:

1. Leave the front plate and just rescrew the matrix. Then the LEDs fit nicely into the printed grid.
   The drawback is that you can't use a diffusor between matrix and printed grid.
2. Turn the front plate around so that a flat surface is created and then resrcew it.
   I chose this option and it looks much better.

Loosen the tiny screws:

Take away the front plate:

Turn and rescrew it: