A method of producing a deflector device for a fuel tank system comprising producing a deflector device prototype from prefabricated components of a construction kit, performing fuel-refilling tests using the deflector device prototype, optionally modifying the deflector device prototype, creating a construction model for the deflector device prototype, producing a deflector device according to the construction model, optionally performing fuel-refilling tests with the deflector device produced according to the construction model and optionally producing the deflector device according to the construction model. The disclosure further provides a construction kit with prefabricated components for producing the prototype of the deflector device for the fuel tank system.