Materials
The better insulator the material is, then the better that material will be for making a thermos. Metals have a high thermal conductivity, therefore you would not want to make a thermos out of just metal. A better option would be something like styrofoam. Styrofoam is cheap and easy to shape, as well as a poor thermal conductor. Fish boxes are an example of an item that uses styrofoam for its thermal properties. Fishermen need to transport their fish without the fish getting warm, so by putting the fish into the styrofoam cooler, this can be acheived. Most thermos aren't just made out of a thermal insulating material though. The insulating material is often surrounded by another material for added rigidity or support. Such examples are metal, plastic, or cardboard.