You don't need a perfect prep machine but you do need to do a little experimentation to get the volumes of boiling and cold water right for the below method. I am suggesting "a quarter" and "three quarters" in the below as a starting point but you can vary this so long as the right amount is being used in total.
You do need an accurate way to measure water - 1ml water weighs 1g, 1 litre of water weighs 1kg, so if you have digital weighing scales in your kitchen you are fine.
Once you have established the right volumes you can boil and chill accurately measured volumes of water and keep them in the fridge - if you have sterile bottles, fill them with the right amount of freshly boiled water and put on the lids, they will stay sterile.
The milk powder is not sterile and does need to be mixed with boiling water but you don't need to use the full amount as boiled. So, measure out the correct amount of powder and then add a quarter of the required water as boiling water fresh from the kettle. Put on a lid and give it a good shake to disolve the powder - it needs more shaking because there is less water in there.
When it is fully mixed you can hold the lidded bottle under the cold water tap for a bit to help cool it down. Then add the remaining 3 quarters of water from the reserves of fridge-cold sterile water.
This isn't failsafe, the best way to be 100% sure is to follow the instructions on the packet perfectly - but as you say you can't always do that. However, the older your baby is, the less dangerous it is to cut corners. A tiny newborn can get extremely poorly very quickly from being given milk that has been prepared in a way that isn't very careful about sterility. Once they are 6 months old and have started chewing bits of carpet/the cat/whatever then it makes less of a difference.