Peter, I took the design from some chap who posted on the forum here and made my own ones. The other chaps are much neater indeed but my manky ones do work fine.
Wozz. Yep, reason i used nuts is no wingnuts in 'stores', but they will be changed for sure. Used 18g but found some 20 afterwards. Should have used that.. next time!
They clamp two pieces of sheet together, leaving a small gap (in this case 18swg) for butt welding. Useful for putting an invisible patch into a panel, provided you can get to both sides.
This diagram i made in Autocad2008 with the aid of my super duper graphic quad core package and card should illustrate what they do. Let me know if your monitor has the resolution to handle such a mega-detail diagram if not i will change it to something that most monitors will be able to cope with.