The bottom edge of the curtains in a VW Vanagon runs on a thin wire rail. This rail has a tendency to flop around and cause the curtains to bunch up. The rail also causes a gap between the curtain and the window, letting in light.
Matt bought some neodymium magnets and slid them into the bottom hem of the window curtain. Using his fingers, he spaced the magnets three to four inches apart. The magnets stick to the bottom metal frame of the window, sealing the curtain against the wall of the camper, and blocking out all light.
Note: Neodymium magnets were developed in part by General Motors in 1982, and are the strongest permanent magnets commercially available. They’re also fairly cheap if you buy in bulk or a few dozen at a time.
Sleeping in a bright area can be annoying. Although Matt showed how he added the magnets to the curtains in his Vanagon, you could do a similar mod to any type of curtain. You might need to make a pocket for the magnets to slide into, or you could glue the magnets directly onto the fabric.