In that situation we use a big black garbage bag. IT weighs nothing, and does the job. But we hike in the Sierra NEvada, where we don't usually get a lot of rain at night.
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Check our our website: http://www.backpackthesierra.com/

Or just read a good mystery novel set in the Sierra; https://www.amazon.com/Danger-Falling-Rocks-Paul-Wagner/dp/0984884963