I've never done a thermos on a hike, although maybe I will start for winter. I would recommend a cup to drink out of, because otherwise you run the possibility of burning yourself on too-hot liquid. However, if you already have an insulated cup, you could get away with a simpler thermos.

How much volume do you want? How long do you want it to be warm? What is the use case (liquid only, or chunky soup)? A travel coffee container might be sufficient.