carrying a gallon of water

For secure and convenient packing with reused commercial items, it's hard to beat those tall, thin Smartwater bottles. Their total weight is a little higher, but they're tougher and easier to distribute as needed.

For a gallon, I have two 1L Smartwater bottles in the outside side pockets (which is what I would normally always carry), plus two 2L soft Platy's which may or may not be completely full. I've just found that it's easier to pack and comfortably balance my pack when it's not all in one big bottle. I would also not want a hard container taking up that volume if not needed.

I know everybody has their own rules about where the bulk of their heavy stuff is in their pack, but I sometimes like to shift things around. When my legs are feeling strong, I prefer the weight around my hips; when they're tired and I'm really using those hiking poles, I'm more likely to put the water toward the top of my back, near my shoulders.

When I'm carrying that much water it's because I know there isn't much to be found on the trail ahead so I tend to split it up among different bottles for the sole reason of not losing it all in case of bottle failure.

For that size storage I use an MSR Dromedary, it conforms to the other items in the pack and in literally decades of use I've never had a failure of either the heavier Dromedary or the lighter Dromlites.