Shoemaker 425 Series 18" x 18" White Step Down 4-Way Evaporative Swamp Cooler Diffuser with Slide-In/Out Damper (Designed to Cover a 18" x 18" Hole) PLEASE NOTE: This product is designed to cover a 18" x 18" hole size; the actual overall outside dimensions of this product measure 21-3/4" x 21-3/4" (see image for further details). Steel galvaneal construction; for evaporative coolers or central blower systems. Modern step-down design; 4-way air diffusion. Countersunk mounting holes for flush appearance with color matching Phillips posi-drive screws; one-piece removable slide damper for off season closure. Soft white finish.