I built a shed recently, making the door from tongue-and-groove decking boards.  Before I could attach the 2x4 braces I needed to clamp the tongue-and-groove planks together.  To fit big tools in the shed this door is 3-1/2' wide, so I needed a couple of big clamps.  Two of these Jorgensen fixtures, plus a pair of 5' black pipes, did the trick quite nicely.