I was in an electronics store browsing the shelves, and I saw a DB9 connector, which is used by the Atari 2600 (and a few others, such as Sega Genesis) as a controller connector port. It was very easy building a tester circuit for the 2600 joystick.
If you use the materials on this page, or any other page on this web site, you do so at your own risk. They are provided "as is". No warranty is provided or implied. I neither guarantee that the materials will work, nor that they will not be harmful in any way.