Abstract: A solar insolation concentrator fabricated from a rigid lightweight (slab-like substrate and a plurality of reflectors. Inclined surfaces are formed in the face surface of the substrate, such as by impressing a heated plate portion of a fabrication tool into the substrate material. The fabrication tool is then used to position the reflectors as they are adhered to the inclined surfaces to assure that the reflectors are aligned with respect to a common focal point. An embodiment of the solar insolation concentrator comprising a plurality of identical square flat back-silvered glass mirrors mounted within recesses within a block of high density cellular polystyrene and a tool and process used to fabricate this concentrator are disclosed. The solar insolation concentrator is also suited for use as a reflector of electromagnetic radiation having wavelengths outside the solar energy spectrum.