You can try putting golf balls in the nesting boxes to give them a hint. The boxes in a dark area can also help. 99% of our eggs are laid in the nesting boxes. As far as space.... Mine are all exterior mounted so that no space inside the coop is sacrficed.
It would be easy enough to section off your long box. My vote is individual boxes so that they have privacy. It also seems that it would be easier to have a nest with 3 walls.