Thanks for the picture – it makes things much clearer. Part of my confusion is vocabulary. The brochure for this boat https://www.sailrite.com/Balboa-27-Sail-Data says it has a 147 pound centerboard. Your saying the boat was without the iron keel threw me off. If the stub keel is still there with its ballast intact, then the centerboard slot should still be there too. If you want to do away with the centerboard, and not replace it, then the easiest thing to do would be to fill the centerboard slot with lead and cover that over with fiberglass. No need for bolts or screws that might cause leaks. No need for other metal that might lead to corrosion or galvanic issues. No need for added stuff underwater that would slow you down. Upwind performance without a centerboard may suffer, but as the saying goes: “Gentlemen don’t sail to weather.”.