Buy a big dry bag from one of the "budget" outfits like Kathmandu or Torpedo 7 etc. Doesnt have to be one of the heavier vinyl type, they also come in lighter weight fabric like coated nylon. I have one that is slightly bigger tham my big Macpac pack, and its lasted for years. When you get home turn it inside out and throw it in the washing machine on a cold cycle for a few minutes and hang out to dry.
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