You might try black plexiglass. It comes in large sheets that are 1/8" thick, is relatively inexpensive, can be cut, drilled and sanded, and is firm and sturdy enough to work well as lensboards. I've used black plexiglass for all my lenses and they work well. I sand the inside surface of the board so that it has a flat matte surface that doesn't reflect light, and I leave the outside surface shiny. I originally got the idea from Bender's website, and it works really well.