27 outlines the boundaries of the wall members 21 23 providing an outside topside and inside face leaving a hollow interior 35 the standards 15 are also molded directly from the sheet 27 and have a hollow interior 36 rigidizers 29 shaped of a linear channel like protuberance molded up from the bottom member 25 communicates the side walls 21 23 with the standards 15 for support thereof fig3 at this stage in the manufacture the base component of the letter block 11 formed from the single thermoplastic sheet 27 is extremely unsteady and may break or crack with abrupt movements due to its thinness thus a resin casting step is next performed to add solidarity to the thin structure the base structure is pulled from the mold and inverted depending upon which side is molded down to form a mold of itself a flexible foam 31 e g a polyurethane foam is inserted into the interior 35 of the side walls to a height of approximately 1 inch where the side walls are of a height of 6 inches the foam 31 is of such a height as to provide a proper flexibility to the sheet 27 in the area of the topside of the wall member as will be described hereinafter the flexible foam 31 is.