Just wear their hair down and use a standard hat
Correct me if I'm wrong, but I think the sikh requirement is to *not cut their hair*, not *wear a turban* per se. They could wear their hair down, plait it and tuck it under their body armour, then wear a normal bullet proof helmet on their head, without violating the religious laws.
I believe there is a steel comb they are also required to wear (the knife traditionally went on the belt, not in the hair), but again, I don't think there is any reason why they couldn't tuck it into a plait.
*I think*. As I say, any sikh reader please correct me!