This is one of the most gratifying parts of building wind-chests and the best method still requires the use of good handplanes. I start with a jack plane for the high spots and finish with a #8 jointer plane. This planing step makes the top and bottom of the windchest flat for attaching the top and bottom with epoxy. I use a Starrett straight edge to check flatness and take one or two passes with a plane and check again for flatness.
