HayBox is the most popular Open-Source firmware for rectangle smash controllers. As of right now, if you’d like to play over USB in Dolphin, you have to grab a .ini file from the repository and drag it into a deep obscure folder inside Slippi’s app data. Ideally, HayBox would be listed along with “Frame1” and B0XX” as default options in the proflle drop-down.

Feel free to make a PR on the Ishiiruka repo to add the ini file or send it to Nikki on discord