I've got the Tommee Tippee flask, so I put water in that and it stays the right tempit works as a bottle warmer too, but that's too fiddly for me!
If i'm going to be out all day, i buy the SMA cartons of milk, and naughty as this is, i'll ask the place where we stop for lunch to stick it in the microwave for like 15/20 seconds, then give it a good shake around & check the temp is ok. Like i say, naughty, i know.
If i'm going out for a couple of hours, i'll feed her before i go out then make up a bottle, by the time she wants it, it's cooled down.