When you are fixing glass in a greenhouse, you may find that the metal pieces that stop the panes sliding down are too small. This will happen if the overlap on planes is too big.
What you can do is make them out of a tin. Just cut strips from a tin (CARE!) with metal cutters.
I can second that. Made some myself this summer after breaking a pane in my GH and breaking the silly little piece of alluminium too. Good tip Davee.