I would go 1 liter or smaller soda bottles. I've had lots of these freeze in cars over the years, and as long as they're not full they haven't burst. The advantage of these over larger containers is that they'll thaw faster when you need them. It would take a long time to thaw a 5 gal plastic jug of water.

I use gallon milk jugs in my chest freezer to take up space and provide heat capacity. This freezer isn't frost free, so they just have 1 freeze cycle, and they still burst about 10% of the time. (Not a big deal in the freezer, as they don't ever thaw.) Milk jugs aren't strong at all and split easily.