There's a little controller adapter that I came across on Ebay which can allow 15 pin gameport PC controllers to work on systems like the 2600 which have 9 pin controller ports. I bought it here: https://ebay.us/ON1DA8
I've spent some time playing some of my 2600 games with PC controllers and I have to say it's really breathed new life into some games I would've not given much thought to playing. I've produced some footage for you if you're interested here: