I use this one

https://www.amazon.com/Magic-Joy-Box-PS2-XBOX-Adapter/dp/B000RLWF7I

Can confirm, works fine with my " DDR Champion Arcade Metal Dance Pad "

I think I used to use this one for PJ64 https://www.amazon.com/Magic-Joy-Box-XBOX-PC-Adapter/dp/B000RLWF7I/ref=sr_1_14?ie=UTF8&qid=1546367319&sr=8-14&keywords=gc+to+usb

I haven't tested it on retroarch but I would assume it should be fine?

I'm sure if you ask in the randomizer Discord they would know about this: https://discord.gg/eebmyE9

I use this:
https://www.amazon.com/Magic-Joy-Box-PS2-XBOX-Adapter/dp/B000RLWF7I/ref=sr_1_2?s=videogames&ie=UTF8&qid=1527783885&sr=1-2&keywords=gamecube+xbox+ps2+usb+adapter

I haven't actually tried the Xbox funxtionality, but for PS2 and Gamecube it works like a charm.

I have this device and it works perfectly for ps2 and gamecube controllers. http://www.amazon.com/Magic-Joy-Box-PS2-XBOX-Adapter/dp/B000RLWF7I. The analog triggers work perfectly under windows, on linux, it doesn't recognize them as analog.