an improved water mattress can be fabricated by a method which involves folding a rectangular sheet of thermal plastic material into a smaller rectangular sheet to form a dual - layered sandwich having three open edges ; thereafter lap - sealing these three open raw edges to form a flat closed envelope , open at its corners ; subsequently cutting specific sized rectangular notches in each corner of the envelope whereby the several apexes of each notch can be separated to form an overlapping slit which is reinforced with a separate piece of vinyl that is integrally bonded with the laps of the slit while it is in its stretched condition to completely seal the mattress . more specifically , sealing the open corners , after they are notched , is accomplished by stretching them over a small elongated oval die sufficient to cause the edges of the resulting slit to overlap slightly whereby an elongated reinforcing piece of vinyl can be lapped therewith and can be thermally welded with these overlapping edges to form a reinforced corner seam when all laps are welded simultaneously . after the last corner is properly sealed , the small , oval - shaped or elongated die can be removed through a plastic valve incorporated in one of the surfaces which is used to fill the resulting parallelepiped envelope forming the mattress .