I think there were big reflectors in the book, but it's been years since I read it. It's the sail that would need to be toroidal and huge. I guess the better point is that, if you can always maneuver the bombs at EXACTLY the right point relative to the sail and they are are perfectly uniformly spherical, and you can orient the sail perfectly before hand, then you can balance on top of the nuclear pin and don't need a stabilizing concentric force.