I've always understood it to be a plate. That way the vector of the forces is in the direction of travel. A parabolic shape would have to resist sideways forces

I've always understood it to be a plate. That way the vector of the forces is in the direction of travel. A parabolic shape would have to resist sideways forces. A bit like this: https://www.centauri-dreams.org/2016/09/16/project-orion-a-nuclear-bomb-and-rocket-all-in-one/