I use some small rubbermaid plastic bottles that I bought at the grocery store. The bottlles are rectangular in shape, maybe 4" tall, 2.5" wide and 1.5" thick. They have large mouth screw top lids that don't leak, but I carry them in a zip log bag anyways. Capacity on the bottles was unmarked, but they hold 9 ounces, enough to mix 2.5 gallons at 35:1. Mark the sides of the bottle with a sharpie pen at 1 ounce increments.
Four ounces per gallon is 32:1, Three ounce per gallon is 43:1. Don't sweat it if you are a little off on your ratios. Pour in an estimate of the oil you will need first, and fine tune as your tank is almost full.