I feel a bit naughty now. After he's had his bath and gone to bed, i boil the kettle and fill a jug to fill 4 bottles of boiling water. let it cool and add formula.
Let the bottles go to room temperature and put the ready made bottles in the fridge, just put them in microwave on defrost setting for 2 mins for a more gradual heating & less risk of hot spots.
I've been making up bottles the night before since he was 3 weeks old and he's 5 months now, and never been ill or poisoned.
Personally i think the guidelines are far too strict, and a little bacteria is always a good thing to help immune system, but im a tough nut who can't be doing with the fancy dancy strict "make the bottle when you need" moto.
when i go out, i boil the kettle, leave it to cool, use a plastic bottle to carry the water, and little formula pots to carry the correct amount of formula for each bottle (there like stacked pots?). Take a few empty bottles and make up a bottle when needed but usually these are emergency supplies, i take 2 bottles straight from the fridge to use if i'm going out one heated ready to cool down within the next 2-3 hours when he'll be needing a feed.
Like i said these are really naughty ways to go about it, if health visitor knew I'm sure i'd be scorned, but like i said, it works for me, and my son is perfectly healthy, and his guts perhaps are getting used to the tiniest amount of bacteria.
HTH. xx