While serving in the US Navy during the Vietnam War, I picked up this tip working in the carpenter shop on an aircraft carrier. It allows you to clamp up a frame with nothing more than string and sticks. Begin by placing the unglued finished frame parts together. Next, wrap stout string at least four times around the perimeter of the frame. Add a foot or so to that, and cut the string to length. Now crosscut eight 1⁄4 × 1" sticks of wood to about 3⁄4" longer than
the thickness of the frame stock.
Glue the frame joints, and place the parts together on a flat surface. Wrap the string as tightly as you can around the frame, and then tie it off with a square knot at one of the corners. Using a putty knife, pry the string away from the frame, and insert two sticks near the center of each frame side. Then spread the sticks apart as far as they will go while keeping the frame flat. This will effectively tighten the string to apply clamping pressure at the corners.