Clamping mitered edges can be a real hassle because they never seems to line up correctly. The easiest way that I’ve found to get around this process is to use painter’s tape as clamps. First set the pieces so that the outer edges are facing up and tape them edge-to-edge. The flip the pieces over so the beveled edges are facing up and glue them together. Complete the process by taping the last two edges together and let sit until completed. The tape removes easily and the glue won’t attach to the tape, making sanding and finishing very simple.