[mark g]

You could try different things. A single piece of plywood would be one but two pieces will control the reaction to "drawing" better by keeping edge ruffles from forming. You could screw through the wood and the metal. In shaping, the metal will stretch the easiest at the center of the cutout but will fail there first too. You might try stretching a little where it's easy then work that back toward the edges in courses. Preserve the thickness at the center if you can.