I too seen that the plans called for a radius. A standard radius would have been fine if the rockers were left orientated as the picture shows. If a weight shift occurs from head to foot, on the original design, the cradle would simply ride high one way or another. If the rockers are mounted across the narrow dimension a weight shift from left to right would cause the cradle to roll over until it hit the stops built onto the rockers and remain there if left unattended.
I'd lean twards an elliptical design where gravity would hold the cradle to center. Any weight shift left to right may make the cradle wain, but wouldn't roll over because of the nature of the ellipse.