Try steam ironing the 2 fuselage sides. Just lay a cloth over the lite ply, and set the household iron to a medium steam setting. Iron one side of the lite ply, flip and do the other. It should lay flat afterward. Lay a couple of books on the fuselage sides to hold them flat for a couple of hours, so they don't re-warp as they cool.

Usually, that will cure warped lite ply problems.