Back when I used to hunt regularly, I would just take my hat off, dust it off, press the top down into a bowl shape, and pour water into that for my dog to drink. Winter or summer, the hat never had enough time to get seriously saturated and I never treated the hat with waterproofing material. I started my dogs this way when they were young and would dump the water when they stopped drinking so that they would learn not to stop until they were full.

Bountyhunter