Sound insulation over tank?

[67 StylesLide]

This pickup has a line of six or eight screws below the back window like there was something hanging from them possible over the tank area.
Is there a source for sound insulation or any recommended upgrades for this area?
My carpet and firewall are already pretty quiet.

[jimmy828]

IIRC if the truck had bucket seats there would have been a tank cover. A carpet that would match the carpet on floor.