If you are wanting a custom built arch try Rail Makers they used to make rails for Pacific Seacraft, they built grab rails and I was very happy with both the price and quality of their work. Rail Makers.com On Shelter Island you can give Steve Harrison a try he does machine work and welding. He modified the sheave box on my boom did a good job, he is located next to Red Sails restaurant.
FYI - Rail Makers is out of Costa Mesa now. He used to have a brother with a shop on Shelter Island but not any more. Harrison Marine is (619) 226-4976. Have yet to talk with him but was referred by Rail Makers office.