72 Demon sails

That cardboard isn't (or shouldn't be) glued down solid-a part of it is not. The end of the clip slides under the area that is not glued.

(edit) To pile on: The cardboard glued to the back of the sail should be a sort of ring shape. The clip goes in the hole in the 'ring' and under it where it isn't glued down. And my '72 Duster had the metal door panel clips originally so they should work on yours as well.