Here's a clamp-free way to assemble frames that I learned from the Seabees before I retired from the U.S. Navy.
First, dry-assemble the frame and tie a strong string tightly around it. Then remove the string, disassemble the frame, and apply glue to the miters. Reassemble the frame, fitting the loop of string around it. Insert two small blocks of wood between the string and frame at the middle of each side. Then slowly slide the blocks to the corners. This applies even clamping pressure across the miters.
—Capt. Doug Dariano (retired) Perrysburg, Ohio