Are you alternating sides with the clamps, crank on this side, then that side. etc? Have you tried 3M packing tape instead of clamps? You can get enough pressure pulling on it as you push it down the get a little glue squeeze-out and a good bond. For pipe clamps try this: get a 24X36 piece of melamine covered particle board get a 36" long piece of 2x4 pine drill 3/4" holes down the exact center of the 2x4 every 6" Rip the board in half screw and glue the pieces to the melamine and you will have perfect sized cradles for the clamps every 6" and a surface the glue will easily clean off