Hi, I bring my ebm out too. I'll usually bring 2-3 bottles in a cooler bag with an ice pack, a thermos flask of hot water (or you can try getting hot water from cafes and restaurants) as well as a small tupperware container.
When i need to warm the milk, I'll pour hot water into the tupperware container and immerse the milk bottle in it.
Reason for choosing tupperware (or any other container that closes airtight) is that in the event that I'm unable to pour away the water after warming the milk (ie in the car), I can at least close the container and pour it away later when i have access to a sink.
hope that helps!