The 5x7 LED panel PCB was designed in KiCAD with 5050 WS2812B LEDs 

The PCB is mounted at the back of a 3D printed diffuser using two layers of white PLA as a diffuser.

The pixels are separated with black PLA. Printing is done with color change, so the outer layer is one closed surface.After some first fails, the white PLA was interweaved into the black PLA by enlarging the second layer. This leads to a robust solution and the white PLA stays in place even when pushing on it directly with the finger.