https://wiki.ai03.com/ is what a lot of people use. Though it's probably easier nowadays to use an RP2040-based ProMicro form factor daughterboard so you only need to handle key placement.

Firmware... QMK: https://github.com/qmk/qmk_firmware/ || https://docs.qmk.fm/porting_your_keyboard_to_qmk

Disclaimer: I'm a QMK Maintainer.