I used a D 9 way connector and serial computer cable for the GPS build I did a couple of years ago. I made a crude cover using smokey grey acrylic so the status LEDs could still be seen.
The computer cable would be rather bulky for a QCX though.
I will look at adding some 3.5mm connectors to my home brew QCX case, not sure if there’s enough room. I guess I might have to remove the key switch (don’t need it anyway).
73, Colin