Not what you ask for, but I had this
old 2xJoystick->gameport circuit built and it's been pretty nice. The downside is that gameports aren't very common now, but on the positive side it also works in DOS.
Before that I had for long time a simpler adapter for the parallel port that works with the gamecon.c driver in Linux. Unfortunately I'm not sure if there are drivers for it for other systems, and autofire doesn't work with it.