Various adapters for sega megadrive (genesis) to atari standard. May not fit because of phisical limitations (be careful):
 
OpenC64MegaDrivePadAdapter is an Open Hardware adapter that allows the safe connection of a Sega Mega Drive (known as Genesis in the US) controller to a Commodore 64 or Amiga computer.

C64 Mega Switcher
With 8bitdo starting recent productions of new quality wireless controllers for the Sega Mega Drive/Genesis, I thought it was time to look into adapting the usage of these with the Commodore 64. Normally a controller of this type would be HARMFUL to your C64 WITHOUT using of a suitable adapter, a great one was made by SukkoPera a while back and I've taken a lot of inspiration as well as lifting a couple of the kicad footprints from his OpenC64MegaDrivePadAdapter. What this project aims to add into the mix is a way of swapping the active port your joystick is connected to at the press of a button, that way you don't have to fiddle around with cables every time you try out a game that for some reason insists on having the joystick in the other port!