When my daughter was little (less than 3 months) and wasn't drinking much, I either made the bottle at home and brought it with me if I knew she would finish it within 2 hours (like I'd make it, she'd sleep in the car on the way to where we were going for 30 minutes or so, and wake hungry so I'd feed her then) or if it was going to be a long day, I'd bring the powder in a pot and a flask of hot water and make it up when I needed it and cool in cold water (usually in a running cold sink). I would sterilise the bottles in the morning and close them up straight from the steriliser. But making them up on the go is a massive pain! We didn't really go out much the first two months, at least not for like the entire day, so I didn't have to do it much, but it only took one time of holding her screaming in a Starbucks bathroom trying to cool a boiling hot bottle in the sink for 10 minutes to convince me I wasn't doing that anymore! Once she was eating a sizeable about per feed (150ml+), I bought the ready made cartons and offered them to her at room temperature, which was fine. That was way easier and honestly I might do that again from the start if I FF next time. If your baby is only eating small amounts and won't finish it all, it does get wasted (and it's more expensive than powdered formula already), but it was well worth it. I didn't go out every day anyway and often I could manage to do the morning feeds before leaving the house, so I only needed to do the early afternoon feed out, which meant it didn't cost too much. It was much easier just to grab a bottle and a carton and go than to deal with portioning out the powder, boiling the water for the flask, and then washing all the extra stuff when we got home.