I don't think you can get truly flat in any hammock. It's just relative. The 2X4 certainly won't prove much for flatness from the bottom but I believe the topside of you will be closer to flat. If you want to be truly flat then you'd have to go back to the ground. The problem I have with U shape is that my knees just don't like to bend that way.
yeah, i was thinking about laying or holding a 6', 2x4 on top.