I am not aware of any covers-including the one mentioned in the post above this one (maybe fiberglass) that are 100% waterproof.
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Geez, I haven't done a scientific measurement of whether it is 99% or 100% waterproof so you have me there. All I can say is that the dust in the bed from those backcountry roads like Cinnamon Pass has not been dampened since we got back to FL with daily rainfall (16 inches last month). That @#!%$ rollup was getting everything in the back wet. The rusty tools were the final straw.