As I see it, Orion has a good principle, but there is one minor adjustment that would make the whole thing more efficient.

Why not make the pusher plate parabolic rather than flat? If the bomb were then exploded at the focus of this parabolic dish, all the force hitting the walls of the dish would be deflected backward, thereby increasing the forward movement. Otherwise, any explosive energy that does not hit a flat plate is simply wasted in terms of propulsion.