A plywood backer glued to wood panel is unbalanced, It will warp with humidity changes, since the wood wants to move and the plywood is constrained from moving by its alternating grain directions. To keep this from warping, there needs to be some way for the wood to expand and contract without resistance from the plywood, One way would be to apply the glue just to the middle of the wood, so the edges can expand without resistance. Another way is to use a backer of wood instead of plywood, just be sure the grain is running in the same direction. The third way is to not worry about the small amount of warping you are experiencing, which I think is what most woodworkers do. As long as the wood is relatively dry, the amount of warping will be small. There is no way to eliminate wood movement without completely controlling the relative humidity of the environment the wood is in. Even alternating the growth rings in a glue up only averages out the warping, it does not eliminate it.