After giving It a bit more thought and research I came up with this design for the shield. I have two Auxiliaries sitting on their own in the cabinet with large hexagonal shields and they look good. The shape of the shields seem to scream out German to me and so I've started to think of them as Batavian.
I will add to them to produce a small skirmish unit of 12 Batavians, all with hexagonal shields (from AandA miniatures) and using Warlord Games Auxiliaries. The shield Design is German also which helps towards the Batavian character. Here they are, the odd couple. At the moment their shields are still blank, but already they look different from the other Auxiliary units.