What we'll have to do is either to sink a lot of concrete blocks into the waters until we have an island to build from, or build some kind of rig to place the nation on. Both of these options will be quite expensive, but I think it would be cheaper to start with the concrete to have some solid ground.
A 10 sq.km island can't be built on rigs. We need concrete pillars and sand / rock.
You could build parts of it on rigs, however that would quickly become extremely expensive.
But we'll need extremely much concrete and rocks

Maybe we could get some concrete companies sponsor this project?
utopia where we build with donations and help each other and only use cryptocurrencies