I can't believe that Phatnoise, who make the cables, won't sell you a Porsche cable even at their usual exorbitant prices. What happens to a user whose cable is damaged for some reason? "Sorry, you have to buy a completely new system." I would try again with them and be insistent. If you succeed they'll probably charge you $150 for the cable.
I don't think Porsche sells the Phatbox OEM, so you won't find any parts from the dealers. So the other option is to make your own cable. You'll still have to find an appropriate Porsche CD changer cable and splice the end that plugs into the head unit with the Phatbox end from your VW cable. AND you have to know the pinouts, with which maybe the more tech savvy people in this forum can help.