I'm actually trying to customize a 'tracktyl' (Dactyl manuform keyboard with thumb trackball)(1). Qqurn has code up at gitlab(2) that can generate layouts with thumb cluster, trackball, or joystick. The code can create .stl files to be used with 3d printers, as well as viewed on-line for sanity checking. Interestingly, the original code was clojure, but migrated to python. Oh! And QMK is awesome for keyboard mapping, macros, etc!

1: https://imgur.com/a/TDE640k 2: https://gitlab.com/keyboards1