I'll second the door idea. Use an interior hollow core door, they are jig built and generally dead flat and true. You can get them from Home Depot, Lowe's, etc. Eyeball it before you buy, just in case, but they are usually perfect. Glue some Celotex ceiling tile to it (spot glue only, to make changing tiles easy) and presto - instant building board. If you rest it on something make sure you have it laying very flat and well supported, so it does not pick up a twist from the supporting surface.


Mark