I Have A Dream Yard Sign Letters - Martin Luther King Jr Yard Cutout for MLK Day Decorations, Martin Luther Yard Sign with Metal Stakes. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. These I Have A Dream Yard Letters full color direct printed graphics are vertically fluted for use with Stakes. Coroplast is used for yard, parking, real estate, and election signs. They are waterproof.