also, calcium hydroxide is Ca+[OH-]2 so it has a net negative charge which means it would have to be the positive electrode that grows calcium

i'm picturing the use of rebar grids  formed into your shape and then weld in a lot of smaller, thinner segments and finally staple over it with fine titanium mesh so it seals up fast and fully, then you would just run it until it's as thick as it needs to be... might need to make it multiple layers thick, but i can easily imagine a bell/dome shaped structure with a long bottom end on it that holds water below the level at elevated air pressure in the rest of the chamber, just a matter of pumping the air in, and it would act to make it buoyant as well, and it just then needs to be firmly tethered on some heavy anchored stainless steel cables to stop it moving around too much and flipping open

still, such a corrosive environment, but then i guess you could have small floating solar panels that maintain a fairly constant electrical current that keeps them steadily accreting calcium rather than rotting the metal