One nice way to think about is to imagine a very light but strong door, and very massive particles. The energy it would take to move the door back and forth wou

One nice way to think about is to imagine a very light but strong door, and very massive particles. The energy it would take to move the door back and forth would be trivial compared to the energy of the system. You can even use a reversible process (e.g. the doors on a spring so the energy put into it to close the door is regained by letting the spring's energy reopen it) to make it use effectively zero energy. Then all the "energy" in the system is in the demon's observation of the system, and not in the actual mechanics of opening and closing the door.