I was tired of trying to find a good joypad. I knew the Genesis controller would work but there is a chance of the controller sending a logical high back to the CIA chip in the Commodore 64 and damaging the chip.
According to this article http://www.floodgap.com/retrobits/ckb/display.cgi?26 , you need to put diodes between the control lines to prevent the high signal from going back to the C64.
So, I soldered up a couple 9 pin serial adapters, wired them to a breadboard, and inserted diodes between. Works great! And there are no weird keyboard errors like the last time I just plugged it in directly...lol
I have attached a few pictures to see my masterpiece. lol
I might eventually solder something more permanent eventually.