to make your own reverse tap, you would just rewire the pins as so:
Duo (pin 4 is the one in the middle)
TG-16
These pinouts match up to each other, if you used 2 pinouts from 2 different website that could have been the problem. I have confirmed that these match up.
this is the cable end connectors. So you would hack the Duo Tap cable, and rewire the same lines to the TG-16 cable. Now you would need a TG-16 connector, those are harder to find that the Duo cables. You also don't want to try doing that without multimeter and solder iron.
However - once upon a time I believe you could buy a cable that would do the exact thing you are trying to do, but those are probably next to impossible to find today.
My next project - extending my Duo Tap cable from 1ft to 6ft!