Howard,
"Cheapest" would be to just get a piece of household fiberglass insulation from the Big Boxes. Its very light. Have wifey sow a big fabric "pillowcase" in something that matches your interior and you'd have a nice big soft cushy and cool rear trunk for a few bucks....
If you are trying to deaden sound, then this adhesive backed stuck-on foam with lead liner is the way to go.. but you pay weight penalty:
Engine Room InsulationKeeps my diesels' noise in the engine room...