Back to my original query.
I found a piece of light gray Corian just over the dimensions I need for about $100 delivered. It's only 12mm thick, but with an additional Y-axis brace underneath it should be fine. An unexpected benefit of my research has been that the sample piece I bought turns out to be quite useful. I made a bunch of parallel bars with it for one thing. I do a lot of operations where I want to cut through the stock a tiny bit. I was using oak or high grade plywood for these jobs. The Corian is much more dimensionally stable. Also, it solvent bonds quite nicely, so I can make complicated fixtures with it more easily than with wood or metal. I highly recommend the stuff!