Well, DH went to town and trimmed a piece of 1/4" Plywood to 18x12. Then he glued the non-warped piece of my warped mat and routered the whole thing. Smoothed the edges - not a splinter in sight. I have not decided yet if I will actually make a pillow bottom for it or simply lay it on a pillow. (Tried that yesterday and it worked like a charm.)
The reason I may not add the pillow bottom is: It is soooo much easier to store as a board.