How do you make formula when out and about?

I mainly express and give breast milk in bottle when out which can be kept out fridge for 6 hours which is great. But when I don't have enough expressed milk I need to give formula. Formula can only be kept for an hour once made and also the powder needs to be put into boiling water. I have bought some of the premade bottles but obviously this is quite expensive in long run to do. Do you take a flask of boiling water and then cool boiled water in a separate bottle? Just means need 3 bottles for 2 feeds.