Here's what you will need
Recycled Styrofoam foam peanuts or packing material made form Styrofoam ALOT of this stuff its easily found since its thrown away alot.
Great stuff expanding foam insulation, the large gap filler in the black can this can be bought at home improvement stores by the can at 4 dollars a can, or this stuff can be bought in 2 large separate cans called true green.
Step 1: Step One: Make Your Mold
Then cut and form a cardboard box that size, The box should fit securely into the space your going to fill.
Then line your mold with some newspaper or other kind of paper.