If I know the feed is imminent I make it up and put it in a thermal bag to keep it warm, if I know it's within the hour really. I sometimes make it a bit too warm so it will be a good temperature when I use it.
If I'm out for longer I tend to take my sterilised bottle, powder in a separate container, a sterile bottle with cool boiled water and a thermos with hot water. I then mix the whole lot eg 2oz hot water, then the powder, then add 2oz of cold water to make it the perfect temperature.
DS doesn't much like cold formula
. I use a perfect prep machine at home so it is always warm.