X3
I keep one around just to seal my tanks when i pull them out for maintenance, keeps the garage from smelling like gas and my wife from freaking out.
Techtip: The GP760/800 gas tanks have a larger rubber cap that fits the outlet on the top as well. I can pull a tank and seal both outlets now which is really handy if your tank is close to full so you don't have to worry about spilling or smelling gas anymore.