From what I've seen the Project Orion and later designs concluded that was the most efficient design. Imagine a parachute (hemisphere) with the place where the explosion happens in the center and the (heavily shielded) capsule behind it, so fully 50% of the explosion is providing forward thrust. You don't have a single tether passing through the heart of the exhaust, rather you've got several going around the outside (like the frame of a cone tent, with the capsule at the apex). This was apparently easier than designing a rigid "pusher plate" with the right shape to capture a lot of thrust.