I did try to make one.
The original has 4 cables, where most with a 3.5mm connector only have 2 or 3.
I heard that 2 of the cables are earths, so believe I made it up correctly, but it’s still not working.
In the Fanatec software it’s fluctuating rather than going on/off.
One off eBay is £16 but not sure if I am throwing good money at bad.