yeh, the underside, cut some packers out of the board, and place them evenly around the board, but tucked in from the edges, this will drop the arch say 5 mill or 2 packers deep 10 mill. This gap gives you something to fill to, to create a firm edge for the underside. If you dont cut the packers short of the edge, when you take them out you risk breaking the edge, being too close, if you know what i mean. Forgot to mention, once the angle beads are up, you should have the same margin/width on both piers. This measurement is the width you cut the hardboard. Also its not a 5 minute job to set it up, but once it is it's easy.