Turn two pipe clamps into one really long clamp
Few us have clamps long enough to pull together a wide footboard and headboard. But you can "make" a clamp that will handle the job. Simply join two 3' pipe clamps with a threaded coupler, as shown. Repeat with two more clamps. Place 3/4"-thick spacer blocks above and below the middle of the footboard and headboard assemblies, as shown, to support the assemblies and the pipe clamps as the glue dries.