Timville has finally put up his 4dapter on Tindie! It lets you use NES, SNES, Genesis and N64 controllers on multiple systems via USB, including MiSTer FPGA, PC, Switch Online, and Raspberry Pi! You can snag them here:
https://www.tindie.com/products/timville/4dapter-switch-mister-retro-controller-adapter/
It has special modes for Switch too for oddball games that do different mappings, like Mario 64 in 3D all stars, and the Sega AGES games