I was taught by guys who worked in the prototype shops for Lockheed and North American. For something straight like a wing panel or the HS, Consider attaching two 3/4" extruded angles Cleeco them to the lower skin, When the HS is assembled for riveting, they should be parallel. The metal will "move" in the process of riveting, but if you monitor the angles, (especially early in the task) you'll see the angles move and have a helper put some preload in for the next rivet or two, until the angles are parallel again.