It probably doesn’t happen often, but occasionally you might have to clamp something together that even you longest clamp won’t reach. While building a new storage cabinet for my shop, I had add onto the end of a panel that was 96″ long to make it 102-1/2″. Of course, I don’t have any 10′ clamps on hand, so I used regular short bar clamps to clamp across the joint at the edges:

Not an ideal solution, since it will only apply pressure at the edges, but better than nothing.

While the metal clamps worked well, my wooden bar clamps made for a much flatter surface for the cross clamp:

Another way to do this is to use four wooden cleats, just scraps, clamped along the edges of the joint. Then as many cross clamps could be used as needed on top and bottom to close the joint. It helps if the cleats are stout hardwood that will resist bending too much.