Over 5 Months later, I did update the Design.
The Server worked flawlessly so far, but there was space for some additions.
First I did mod the MiniATX Powersupply and build in an Switch to put the Powersupply Fan into 12 Volt (Fast and Loud) or 7 Volt (Slow and Silent - but enough throughput) Mode. From that on, the server was a LOT more silent.
Also, the HDD Cage connection to the ground of the Case was damped by screwing it through a thumbsize layer of rubber. That helped.
But the biggest change in this revision was the attachement of an LCD Display.
It was connected through the LTP Port of the Mainboard and with an special piece of Software, it was able to display Live Data like CPU and Ram Load, Network Interface Load and a lot more.
The display holder was created out of metal and did not only hold the display, but also the display controlls (brightness, contrast and display on (backlight on), display off, display on (backlight off)) and an 12 Volt / 7 Volt Mode switch for the big 120 mm Fan.
And after that,.. there was silence 😉