Atlantic Towers out of new jersey they will make you one for about $1100. I have had them make 3 of them up for our boats. A 84 searay 21ft,76 searay 30ft. and a 83 31ft. searay. All out of aluminum and just bolt on. Check out the cobo boat show. I have talk to them for last 8 years.
On ebay I was looking under marine exhausts and came across other items for sail and they had arch's.
Great Lakes Skipper LLC was the sellers. They had one fibber glass for a that may fit. Also was a aluminum for a searay. the stainless and aluminum can be bent for the with of your beam perty easley. You can call them at 414-431-6601.