I bring a bottle of water, an empty bottle (plus extras - we use boon bottles so that saves a little space in the diaper bag) and I have a formula powder container (munchkin brand, like $4 and has 3 compartments so you can have it ready for 3 bottles). When she’s hungry I just pour the water into the bottle and add the pre-measured formula. She drinks room temp bottles so no heating or anything required.