I have updated my web site at http://joystick.world3.net. Plans for the Retro Adapter are not available!
Currently supported are Atari/Commodore style joysticks (up to three buttons), Sega Saturn, Mega Drive (Genesis), Master System, Nintendo Famicom (NES) and Super Famicom (SNES) controllers.
Six button Mega Drive controllers are not currently supported. I know how to do it, the problem is there is no good way to detect them. I may or may not add some kind of automatic or manual detection. It could be as simple as holding down the start button as the adapter is connected to USB.