A do-it-yourself kit for retrofitting without special tools protective weatherproof cladding on an existing door or window frame assembly. Three elongate one-piece sections of extruded plastic are prefabricated in transverse cross section to match the profile of the exterior exposed surfaces of two sides and the header of the frame assembly. Lengthwise portions of each section are folded to grip and secure under friction door stop or sash guide surfaces of the frame assembly. Tabs extending from mitered upper ends of the side sections underlap mitered ends of the header section. Templates are included in the kit for cutting the lower ends of the side sections to match the profile of the door threshold or window sill.