If you want to change the thermos into an open energy system, you have to remove the lid. Once the lid is removed, the energy is no longer contained inside the thermos bottle. From the bottle, the energy dissipates to the environment.
The definition of an open system is one that can exchange heat and mass with the surrounding medium. A closed thermos cannot exchange mass or temperature with the medium. By removing the lid, it has the possibility of exchanging mass and energy in the form of heat with the medium in which it is found.
