Finally got all the parts. Ordered M2 screws, spacers, wires and MCUs from banggood.com and scavenged some micro USB cables that were about to be thrown away at work.
Had to enlarge the holes in the middle layers to fit the spacers but luckily I know a few guys that own a makerspace where I could borrow a drill press
There were so few keys so I could use a dedicated digital IO for each instead of bothering with a matrix. The thin wires broke a few times so I had to resolder a couple of them
USB plug fit perfectly in the hole with a minimal amount of filing. It fit so tightly that I didn't even have to attach the Pro Micro to anything inside the case.
Wrote my own firmware using Arduino's HID and keyboard library. Originally I had planned to map the keys to F13 through F22 since I thought they would be unused. The keyboard lib didn't support sending those codes though so I copied parts of it into my own project and hacked it a bit. Unfortunately it turned out that those keys have been mapped to extended functions in Linux and I didn't want to start launching programs by accident so instead I mapped them to numpad 0 through 9.
Bought two different colors for the two macropads I'm building
Experimented a bit with mixing colors but I thought it looked bad on the keyboard since there are so many different materials already.
Finally done!
Wrote a symbol table so that the key codes are translated to unicode cat face emojis instead of boring numbers. The complete layout is:
🙀🐱😸😹😺
😻😼😽😾😿
(if your OS + browser supports unicode)