Stoppy Ultimate Door and Window Stop
A flexible and handy little device, wedging a comma-shaped Stoppy under or between any door or window keeps them in place. Made of weatherproof materials, the stopper works indoors or out and grips almost every type of surface.
Available in a variety of colors, Stoppy sells in the U.S. from Bed Bath & Beyond for $8 and online from Under The Roof Decorating, which also lists international retailers.