Retired Fire Chief

After showing my husband pictures of barn quilts he now wants me to make one to hang over our big red barn doors. I will design the block and he will even help me paint it, but the problem is, he isn't sure what material it should be made of - I suggested exterior plywood but he said that warps and will peel, which is correct, it is designed to go under exterior sheeting and finish siding. Short of steel, what would you suggest?

Thanks in advance, love all the help I find here!