How about not mixing it yet. I have a munchkin formula dispenser that I fill up and then take bottles and a bottle of water with me. I rarely use the bottle water because I am always able to find warm water to make the bottle. My LO is picky about the temp of the formula.
This. At home I always have premixed bottles in the fridge, but when we are going out, I just put enough powder in the bottle, and then bring a separate bottle with remeasured water in it. For us this works the best, since giving my LO formula that has been refrigerated and not warmed will cause her belly issues.