A light cover, such as a prismatic lens, includes a generally planar rectangular floor with curved walls rising from a periphery of the floor. The curved walls terminate in outwardly turned flanged-type lips create an upper periphery or opening of the lamp cover. A pair of parallel or spaced apart rails extend around the upper periphery. An interstice is formed between the pair of rails. When the outwardly turned flange-type lips are urged against a sealing or washer-type material, the sealing or washer-type material is urged into the interstice thereby forming an improved watertight seal.