Try to find pictures of the helmet. FRONT view, SIDE view etc..., it is the same helmet used in World War 2 & Korea, the one in Vietnam had a CAMO cover, get ALL of the reference pics you can find, here is a good site for stuff from Vietnam: https://www.vietnamgear.com/kit.aspx?kit=25, you are looking for the M1 Helmet. Google it. Another thing you can do is, make or get a model of a human head, and use it as reference to model the helmet on it.