21st Birthday Yard Sign Decorations 18" x 24" Personalized - Coroplast 21 Year Old Birthday Party Happy Twenty First Birthday Yard Decor with Metal H-Stake. Economical and durable, our Coroplast Yard Signs are a staple of outdoor advertising. These outdoor 18" x 24" 21st Birthday Yard Decorations Supplies for Girls full color direct printed graphics are vertically fluted for use with an H-Stake. Coroplast is typically used for yard, parking, real estate, and election signs. They are waterproof.