A method is disclosed. The method includes forming a groove in at least one of opposing planar surfaces of a cellular thermoform plastic substrate. The groove is formed at an edge region of the substrate. A stiffener is positioned in the groove, and a fiber matt is laid on the at least one planar surface containing the groove. The stiffener and fiber matt are embedded in a thermoset resin. The thermoset resin is cured to form a fiber reinforced thermoset plastic reinforcement. The substrate is then cross-sectionally cut to form a profile. The profile can be further finished to form a beveled siding for buildings.