Amazing. I was planning to do something similar into the Jakks 10 in one. But even before destroy, drill, wire up something, I decided to search about it.
Your PDF about how to do it is perfect.
Already bought an old Atari 2660 clone in Brazil (yeah .. due some restrictions at the Atari time we had tons of cheap clones .. not that cheap), and an raspberry zero to adapt inside.
Downloading your Ultimate Joystick image.
So when everything arrives will be a quick job to put all together.
Quick question ... that Ultimate Atari Joystick is ready to go ?
Just unpack, save on a SD and use it? Already configured all scripts and butons based on your PDF?
Of course adding some extra roms and box art.
Once again .. bravo. Excellent job.