My order has arrived and I have assembled it. It's not yet fully working as expected, but the back buttons and the power switch are now working reliably. Though one back button didn't work shortly and I couldn't find out why - after reassembly, it was OK again... I am considering a 3rd version to dent out these issues as well. It's a bit annoying since it costs 30€ to make a try, but it's fun at least to see something working.
The screen buttons aren't working so well - the bottom ones are OK but I need to redo the button sizes again. The upper buttons on the screen aren't going to work that easily though. The problem simply is that the joysticks would collide with any button mechanic. So I probably will only provide the bottom buttons in the final version.