If you trulywant differential with a single servo you will have to use the round "arm" and drill holes offset from 90° by 30° (120° apart). Or, youcan use the 4-way arms and offset the contol horns so the holes are not over the hinge line (not recommended). You coud use the 6-arm head, but using twoarms 120° apart .

For best results you want more up throwthan down, by the way.