You are correct in that the "clip" just holds the magnet in place aginst spring pressure until the drum has been installed,then it "floats" on the magnet arm.
There should be a small (about 1/8 inch) hole drilled in the end of the magnet mounting arm.
In a pinch you can cut a short piece of nail to fit into the hole to hold the magnet on until the drum is installed. Be sure that the pin is stiff enough and long enough so it won't fall out after the drum is installed.
If your experence is like mine, every RV shop that I went to said the "clips" weren't available as a seperate item. (they just wanted to sell a complete magnet assy.)
I called Dexter (you have to know what brake assy that you have), they sent me 6 clips @ no charge.
May you have fair winds and following seas!
2005 Dodge, 2500, auto, 2wd. Pac brake PRXB. CB, Max Brake Brake Controller, Rhino Lining, Aero 60 gal. replacement tank.
1998 Hitchhiker ll, mod#31RLBGBW, RBW L'tl Rocker Hitch, Generac NP50 G generator