When I'm at home, I boil the kettle in the morning and fill as many bottles as I need for the day, to about 3.5oz, put lids on and leave to go cold. Then when she gets hungry I top one of the bottles up to 5oz with boiling water and add the formula. This seems to get it exactly the right temperature to give to her.
So, when I go out I'll have a bottle with 3.5oz of cooled boiled water, the right amount of formula in a container, and a thermos flask of boiling water that I can use to top it up to 5oz when I need and just add the powder to that. Maybe I'm overcomplicating but it's worked so far for me. If I'm out for a while and the thermos cools down, and I'm in a cafe or someone's house I can just ask them for some boiling water to top up the bottle with.