Get a reasonably heavy plastic bag that'll cover your camera, like a 1 gal ziplock.
Take off your lens hood and stick the camera in the bag so the lens is pointing into a corner. Pull the bag tight so it stretches a bit around the outside of the lens.
Use your fingernail or a plastic knife to tear away the corner of the bag then refit your lens hood to hold the bag in place.
Pull the bag around the camera to see where it lays against the viewfinder then tear a small hole there.
Remove the eyecup from the viewfinder, it should just slide up. Fit the small hole over the viewfinder and replace the eyecup.
Ta-da! You've made a field expedient rain cover.