I just completed a custom y-adapter cable for a friend. I used some neutrik connectors; a 3.5 TRS to two 1/4" TS. I used some paracord for the sleeve and a bit of heatshrink for the y-split. It came out very good and feels pretty solid.
The connections were soldered and wrapped electrical tape around the paracord in the center beneath the heatshrink to add a bit of stability to the y-split paracord junction and also some electrical tape around the paracord where the connectors crimp the wires. This gave it a nice snug crimp without being too forceful on the wires and allows some strain relief from the paracord being crimped and not just the cable.
Have a look. If you have any questions or comments please let me know below. :)