Where/how does it mount? If it's basically a cube shape, there are off the shelf options.
DO YOU KNOW OF A MANUFACTURER WHO MAKES A FUEL TANK OF THE SHAPE UNDER THE CAGE LIKE IN THE PICTURE?And by the same logic, when one installs the fuel tank in or on the vehicle, it becomes part of the vehicle. You make the argument true with the firearm analogy. Attach something to the lower receiver, it becomes a defining part of the overall firearm.
You also make the statement true with the watch analogy, one can't choose to hold the gasoline in their shirt pocket or cupped hands, where those containers would not be part of the vehicle. It has to be in a suitable container and that container is part of the vehicle regardless of method of attachment to the vehicle.