From my notes, Open-top vehicles include the following:
Bren Carrier, Gun Truck, Jeep, LVT4, M3, M3A2, M7, M8 AC, M10, M18, Marder I, P4 Milan, P4 12.7, PzJäger I, SdKfz8, SdKfz251, SdKfz251/9, Sig 33 (HoB version), Somali technical, Su76, T13-B3 and all choppers.
I'm wondering if the Sd 222 (from NOSB) should be added to this list. The most protection it had at the top of the turret was an anti-grenade net.