Great Job on the Weathered Look!
I found that if you leave No.2 Copper or strands on metal or wood it turns green in areas . .in about 2 or 3 days if you wet it and let it sun dry. Another method is using a chlorine or bleach to speed up the process of the oxidation. . the result is a stain.
You could strip some wire and make a circle around your porthole?
Looks Similar to This:
This method works best if you rough the area with a wire brush.
Additional Information: Here