Regular old houshold bleach bottles are great. Now I know this sound unapealling but bear with me. The jugs are thick, the lids fit really tight (and you can still wrap them in duct tape for extra strength) and the very small amount of bleach left in the bottle keeps mildew, fungus and bacteria from growing, but has no adverse effects on humans. The moulded handles also makes it easy to tie them to the outside of your pack. All you have to do is stop at a couple local resteraunts and ask them to save the bottles for you, so you don't even have to buy anything. After you have your bottles, rinse them out really good with hot water and let them dry out completely, bleach will evaporate almost completely off a non-porous surface. Fill them, cap them, and tote them to your cache site. The best way to find your cache site is to find the general area where you will need to resupply. Hike there with your cache and mark an easily identifiable landmark on your map or build a small cairn (also marked on the map). Pick a bearing on the compass and walk, as best you can for at least 150 paces (the further the better). Wrap the water and anything else in the thickest plastic bag you can get your hands on (resteraunts are a good place to find these for free also). Bury this at least 18 inches (this means 18 inches of dirt on top of the bag) Then pile the extra dirt somewhere nearby and try to hide the evidence as well as you can, spread leaves, twigs, rocks etc.. around. Follow your bearing back to the trail and mark the direction on the map near the landmark or cairn. You should be set, but remember animals can be persistent and very clever, and humans can be inconsiderate, NEVER rely solely on your cache, you should carry enough water to make it well past your resupply point. When you return to collect your supplies, take the same pains to hide the spot again. Fill it in with the dirt pile you saved and cover it all up again. Remove the cairn after it has served its purpose, try to leave no trace. Keep us posted how your plans are going, I am also thinking of hiking the AZ trail and its nice to hear how others are dealing with planning dynamics.