I've got a thermal bag which can be used to store bottles hot or cold, for about 4hours. I have made up feeds in advance and stored them in the fidge for 12 hours max, but now I just fill the bottles with boiled water and store in the fridge for up to 24hours. Then add powder and heat when required.
If i'm going out and won't be able to heat up a feed I put boiled water in the bottle then cool it a little and keep it in the thermal and take a small box containing the right amout of powder then I can mix it when needed.
Depending on which formula you're using you might be able to buy small cartons of ready mixed formula which can be served at room temp, I know SMA and C&G do it, but haven't actually tried them!