My preference is to laminate two thin pieces of conventional 3/16" stock, cross-grain and heart-to-heart, and then cut a rabbet along the edges with the shaper, and route out the center area of the back side. That way, you end up with more of a light trap around the edge. Another option is 1/8" plywood, available at many hobby shops, but usually not carried by Home Depot-type stores. Again, laminate for greater strength and the light trap at the edge. Any tight-grained hardwood is OK.