I remember an episode of mythbusters where they were making model airplanes I believe out of concrete. They used a styrofoam mold and layered the concrete on. Then they used I think rubbing alcohol on the foam and dissolved it away leaving just the concrete. Has anyone tried this or think it might work? If it does dissolve the pink insulation foam that would certainly remove the buoyancy issue as long as the concrete structure is rigid enough to hold on its own.