The JJC Disposable Raincover for DSLR is great in an emergency. Simply slide over the camera and lens and use the draw-card to cinch it tight around the end of the lens. It's large enough to take a lens up to 400mm long (that's 400mm physical length, not necessarily 400mm focal length). If desired, make a hole for the viewfinder and use the camera viewfinder eyepiece to keep it in place. Weighing around 30g, the JJC Disposable Raincover packs into the smallest of places, so you can always have one with you.
