After the testing was done and some fiddling around on how to arrange the speakers, display, Raspberry, USB Hub,.... and so on, I decided to take the first step and cut the ammunition box to hold the display.
As you can see my cut was a little bit off,... but with an angle grinder you can't expect brain surgery precision (at least not from me).
Then I got some motherboard mount screws, screwed them in and turned them into rivets (hammered them flat so they won't fall out)
After some tedious drilling and rounding off the edges with a counter sink I secured the speakers with hot glue and display with Raspberry Pi on the back onto the mount screws.
I made another hole in the back to have the option to use an external (USB) power supply instead of the powerbank.