You have lost your gas cap and need something to cover your car's gas tank to insure gas fumes don't escape, leading to air pollution, and your gas does not evaporate at a quicker rate. There are at least three steps you can take, depending on where you are and what resources you have when you discover your gas cap is missing.
Buy a temporary gas cap at your car dealership. Be sure to get the right type and make sure it fits properly.
Tear off a piece of aluminium foil large enough to cover your gas tank opening. Place it over the gas tank opening. Secure with a heavy-duty rubber band.
Ask the attendant at the petrol station for a spare cap. Many attendants keep the caps that have been left behind.
Place a thick rag or a thick washcloth around the gas opening and secure tightly with a heavy-duty rubber band.
Keep a spare gas cap in your glove compartment.
Fuel systems are meant to be sealed to work properly. Make sure to replace your gas cap as soon as possible. Always be cautious as putting anything foreign over your gas tank can be extremely dangerous. Due to fuel vapours ability to burn violently, driving without a gas cap puts you at great risk for fire. (ref 2)