Here is a link for a pack cover that will also serve as a gear hammock and water bag: http://www.tothewoods.net/HomemadeGearPackCoverGearHammock.html

Jeff makes it 47x58. I make mine much smaller, 38x38 or so. All it is is a rectangle with the corners cut off to make an assymetrical octagon. Hem the sides, run a drawstring through the hem, put a cordlock on the drawstring. done.