I've seen nylon stuff sack style dry-bags at Walmart for $10 for a three pack. They are made from woven nylon with a roll top closure and are supposed to be watertight. The biggest of the three appeared large enough to hold a pair of hiking boots or maybe a bit bigger.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)