The only cover I know that works with it is the OEM soft cover, which has a rail-less implementation as they're basically just clamps. As for flex, I think it's more to keep the bed square/true so it doesn't warp to ensure the doors can still close. If you're carrying lots of payload or going through rough terrain, I can imagine some articulation scenarios where things could rack or flex and cause the doors to misalign. Although I would imagine it would be the same issue with a regular tailgate? Or maybe the extra weight of the steel brace for the doors would cause some warping of the bed if not properly reinforced?