A piece of 1/4 inch black acrylic cut to the OD of a pacemaker board opening, corners rounded to fit into the opening, back sides routed down so that the center portion just fits into the bellows throat and the front edge just under the board locks ( approximately 1/16 inch thick lip ), back side sanded so that it has a flat finish, and center drilled to the shutter mount makes a very nice lens board that is very rigid. It is a little more work than foam core and the center portion going into the bellows opening eliminates the need for the outer lip light trap.
http://www.graflex.org/helpboard/vie...ght=lens+board