Here is a wiring diagram taken from Direct Pad Pro98, it is basically identical to that used in SNESKey, with the addition of an extra 2 data lines, for extra power. As you can see, it is QUITE simple in construction.  5 diodes (preferably germanium) for protection purposes, a DB25 male connector, and a SNES connector (or bare wires from a joypad or extension cable.) About 20 minutes in construction time is my guess

For a more detailed look at how to build your own adapter, check out my "how-to" guide.

If you're looking for some real hardcore info about how the SNES joypad works, check out this FAQ I came across on one of my many web hunts.

Also for those interested, I have included an online copy of the SNESKey documentation, because the official web site is no longer online.  To view the documentation click here.