For just a gallon or two, I'd go with Nalgene water bottles and would keep them corralled and contained in a soft-sided cooler (I have a square soft Igloo that holds several 32 ouncers).

I camp a lot and keep several mostly full Nalgenes in my freezer for use in the camp cooler. It's efficient, tidy and has saved a bundle on bag ice during 2 and 3-day trips.

None of my Nalgene bottles has ever leaked. Some of mine are a decade old.

Either the 64 oz rectangular "Space Saver" BPA-free (this would be my choice for car storage - just 4 equals two gallons):

http://www.campmor.com/outdoor/gear/Product___80924

32 oz cylindrical

http://www.campmor.com/outdoor/gear/Product___80880

or 48 oz cylindrical

http://www.campmor.com/outdoor/gear/Product___80883


Leave a couple inches from the top of the bottle for expansion.