What's the best way to store CPR playfields so they don't bow/warp? I see folks with their playfields out of the boxes and stacked on a shelf with rubber spacers in between them but I haven't got room for a setup like that. They're staying in their boxes so they stay protected, should the boxes be laid flat on the floor under my pins, up on end and flat against the wall, or on their sides and flat against the wall?