How can I make homemade miso mix ready to eat?

I know there's many instant miso mix out there. Instead of filled preservative instant stuff, I have a fresh box of miso paste.





My office doesn't have microwave or stove, so it would be inconvenient for me to bring miso soup. By the time for lunch, it's already cold. I don't have those fancy thermos lunch box either, so lunch box is out of question. Even though it looks bad, my office has hot and cold water dispenser.





I wonder if I can make mix miso paste at home that I can bring to the office. I just have to add hot water into it and bang! A bowl of hot miso soup is ready for lunch.





I'm thinking about boiling miso paste with dashi and water and let the whole mix to be reduced to paste again so it's already cooked, but I'm not sure if it's safe to be mixed with hot water and ate later on.





Any idea?|||Ein, I wouldn't know where to start with that, for the price of them these days you can buy one those wide neck thermos, pretty cheaply.