They sell diaper bags with insert for a bottle and frozen pack, like a lunch box. That's what I used and you just have to use it cold unless you're somewhere you're able to warm it up. DD2 never has a problem drinking cold breast milk, lol.
Our fridge always got super cold so I'd just pull it out of the fridge and by the time he was ready to drink it was still pretty cold. I have a diaper bag now that has a bottle holder that keeps them cold and don't need ice packs. If say it would depend on how long it would be before needing it.