Today we've come forward. The front now is closed and the control panel (still without holes) fits onto the bracket. In addition, the speaker panel is assembled with the pre-drilled holes. The appearance from the box is now more like a video game machine ;)

Also, I've modified the PC. Since I don't want to open the maintenance door to power-on on the pc, I've dismounted the original on-off button and connected it via cable to a black game button. The button later will be installed underneath of the control panel, so that the PC can be easily turned on from outside.