We invented these for a river trip.  We wanted a way to carry water and also wine in such a way that we could stuff it all inside a hatch while paddling, plop it in the cool water to chill when in camp and/or hang it from a tree for easy access.  We used empty rinsed out wine bladders for water—they do not impart an unpleasant taste to water like the plastic collapsible water containers from the sporting goods store, and we used full wine bladders straight from the box for the wine.  The hole down in the corner is, of course, for the spigot.

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Here is Sandra at Army Hole during the last Texas200 using one of these covers to pour a glass of wine so we could celebrate having made it. It was box wine and it was a plastic glass, but you know what?  At that point, we did not care.