Borteese's Je Sus is a well-documented open-source 49-key board.
KBD.news Published January 8, 2022
To create the tenting, Borteese came up with a custom keyboard mixing smaller PCBs and handwiring.
This has a five-part PCB and is almost fully hardwired. I also designed the case and 3d printed it. To program it, I use an Arduino Micro. This keyboard can access almost all the keys on a full-sized keyboard using layer button (the menu button next to the left spacebar) – Borteese.