I need to make one right now, and will probably use scrap aluminum plate from an electronic "rack panel". I've made them in the past by gluing together two pieces of fiberboard, one slightly smaller to fit the internal cutout and prevent light leaks. The epoxy method would probably work, but seems like a lot of work. You could certainly make a mold from 2-part RTV and cast a new lensboard with epoxy. Be careful about the filler, as many dye type fillers transmit UV and/or IR! Painting it will work, but chips in the paint might leak light. Actually, that method might be quite good, but I don't have an original lensboard to copy.