Instead, use an insulated container (tall or short) and preheat it before you leave home.
Pre-Heat Your Lunch Box
Let's say you had lasagna the night before. Heat it up at home in the microwave or toaster oven on a microwave-safe plate.
While your lunch is warming up, boil a kettle of water and fill your empty lunch container with the boiling water. Allow it to sit for about 5 minutes and then pour it out. Adding warm food to a warm container makes much more sense don't you think?
We've kept warm food, well... warm, for up to 6 hours before with this method and it means no waiting in line to use the smelly communal work microwave. It also means that your break time can be spent doing something other than heating your food and checking it. Heating it some more and then checking it.
You can sit back and catch up with some blog reading while enjoying a hot, home cooked meal!