Cheap, simple, and fills in any gaps in between the tank and frame: hot water pipe insulation. It comes in long sections split down one side and costs very little.
It also removes easily and will not mess up the finish much.
Race teams like Nascar and other series that still use lug nuts use it (windshield glue) for their wheel nuts, they glue the lug nuts on the wheels with this goop to speed up pit stops.