Not sure that I have any great ideas for a T-Shirt, but I do have another idea -
I am a member of the Porsche Club of America (PCA) and as part of our membership we get little vinyl PCA decals we can stick on our windshield. I would love to get a CGOAMN vinyl decal that I can stick on the windshield of my soon-to-be-classic Glastron!
Also, Porsche clubs do decals/emblems for various events that you can display on your car. Maybe whatever logo you decide upon for the T-Shirt you can do as a vinyl decal with the event year and people can collect them over the years and display them on their boats.