How do you create an even, symmetrical concave in a hand plane? I suppose you could shape some divinicel and then lay-up some okume or pawlonia veneers over the divinicel and vacuum bag it, but they you still have to shape the divinicel with an even convex shape. The other option is to hog-out some thicker wood with a surform. If you do that, do you use the the depth probe of a set of calipers to measure into the concave from a straight-edge laid across the hand plane at various points to see if the concave depth is the same on both sides of the center line? Are there other methods?