techra:
we are not allowed to insert anything inside the fuel tank
Does that include electrified metal plates? This sounds like a highly dodgy approach to me. Are you aware of any mass-produced fuel level sensing systems that use this approach, perhaps on a bike? If so, you could grab one of those sensors or make your own to a similar design with some confidence that the principle was sound. But a DIY sensor constructed of live metal plates immersed in fuel and surrounded by vapor sounds like an accident waiting to happen.