Such as for the Gunner/Commander stations, try this material for recreating realistic ones. It's a thin decal sheet and when cut-down to size, could make really nice ones.
http://cgi.ebay.com/Echelon-1-35-M113-Periscope-Pink-Inserts-356093-/400206237414?pt=LH_DefaultDomain_0&hash=item5d2e268ee6