The diamond gens are pretty secret already and adding more than one generator at mid would be pretty game-changing, while single islands are too small to hide something. When you find the location of the "secret one" it'll always be there and not be secret anymore. Because you can't break blocks.
All diamond gens are "secret" initially because no one knows where they are. Then they find out. They remember. Same thing for the "hidden" ones.
Sorry to say this but, -1. Bceause hidden gens can't exist. You can make them less obvious but not hide them. It's just not that easy.