I have several small to medium size Sealline dry bags that I use inside my primary packs. I have rain flys for my packs that get used for backpacking/climbing/etc but I use the Sealline bags when I have more than rain to be concerned with.
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"There is not a man of us who does not at times need a helping hand to be stretched out to him, and then shame upon him who will not stretch out the helping hand to his brother." -Theodore Roosevelt