The headrest on the co-pilot's seat at the helm broke off. At the time it looked like a very expensive disaster. I suspect that it gets used as a grab rail when people move about the boat and the design/engineering is a bit lacking. To fix it, I removed the seat from the boat and took the seat back off. The plastic part in the photo is attached to 3/4" plywood within the headrest and the plywood within the seat at the bottom.
I removed the staples from the headrest vinyl cover and managed to pull it off without breaking the stitching. I made a replacement part for the broken plastic. I considered buying a piece of Starboard but, as I usually do, I relocated a plastic cutting board from the kitchen to my shed. After cutting the plastic to size I beefed it up with 3mm aluminum cut to the same size and reassembled the headrest and screwed it in place. I found it very difficult to get the vinyl back on and stapled down but a local canvas/upholstery shop did it for $10. It's many times stronger than the original.
Just in case someone breaks the same part.