Formula fed babies on the go?!

My baby is formula fed as I didn't produce any milk myself. This is fine it works for her. When we are home I use a Tommee Tippee Perfect Prep which is great for while we are at home.

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My question is, when we go out for long periods of time what is the best way to prepare her bottle? 

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She is currently on a comfort milk which isn't best to be ready made and fed whilst out. I read that water must be no less than 70 degrees in order to kill bacteria. Would I be able to boil 1ltr of fresh water, leave to cool to 70 degrees and then transfer to a flask to keep at 70 degrees while we are out, then pour the required amount into a sterile bottle add correct formula, shake, cool and feed? 

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Or if anybody does it any other way please let me know!