I am planning on making some lens boards for my Graphic View cameras, and was curious to know if styrene modeling plastic of the correct thickness would be an acceptable material to use.

If it doesn't flex and is totally opaque, I expect it would work well. If you can mill it without the plastic melting, it shouldn't take too long to fabricate a board, and once you got the first one done, doing others would be a piece of cake, especially if you have access to a CNC setup.

I use two pices of Midwest #5305 - Baltic Birch 3mm
like ebay #280791723478
easy to cut with a square & a utilyknif one outer & one inside as for a light trap

I've used black ABS and Acrylic to make lens boards for various cameras taking lens boards from 4 inch square to 6 inch square and they worked fine. I prefer the Acrylic over the ABS. The plywood Banjo mentioned is great also and available at many hobby or arts & craft stores. The biggest issue with thickness is will the shutter mount threads clear sufficiently for the lock nut to mate fully.

Cutting the outer edge down so it fits into the camera slots and leaving the rest to fit inside the front standard/bellows throat helps create a light trap.

Flat black paint on the inside (plywood) or sand the gloss off shinny plastic is necessary to eliminate unwanted reflections inside the camera.