Abstract: A low labor-intensive method and construction for joining first and second planar sheets of plastic at respective edges thereof at right angles to one another without the use of adhesives or solvents. A first lip is formed extending along the edge to be joined of the first planar sheet at right angles thereto. A second lip is formed extending along the edge to be joined of the first planar sheet at right angles thereto. A plurality of guide pins extending outward from the first lip along the length thereof and perpendicular thereto. A plurality of guide pin receiving blocks are formed disposed between the second lip and the second sheet along the length of the edge thereof and disposed to be opposite one of the guide pins when the first and second edges are abutted with the first lip abutting the second sheet and with the inner and outer edges of the second lip parallel to the outer edge of the first lip.