Hello Andre,
I can recommend plywood for DIY use. It costs practically nothing, is easy to work with and is stable enough. I use 2mm plywood in my DIY project from the beginning.
In contrary to the MIDI hardware you can see in the photos above, I now use an Arduino or Raspberry Pi Pico controller for each device.
I wish you a lot of fun with your project and if you have any questions feel free to contact me.