Finally, the screws have been delivered.
The new 20mm long screws go straight through from the X735 board to the heat sink to the RPi4 to the X825 board. Fits perfectly with some air space over the heat sink to.
Time to insert everything into the chassis (X825-C8) and fasten everything with screws from underneath. Connect the switch to the X735 as described here. I have a momentary switch, so it should be connected to the 4-pin connector in the picture below. Had it been a latch switch, it would have been the other connector behind it.
Attaches the last part of the chassis and also the small USB bridge from the SSD board X735 to RPi4.
I also inserted a bluetooth adapter in one of the USB2 ports.
It turned out really nice if I may say so myself.