I can mail you a cutout of 16 g that you can screw down if you can't find one. I work in a metal shop,have tons of scrap. Just message me. Otherwise,roadside signs are good,but most are aluminum and will warp because its a soft metal. Look up metal suppliers or sheet metal shops in your area or the nearest town over,they'll gladly cut you what you need and sell it cheaper than home depot would,maybe even free