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.
Step 2: Step Two: Filling the Mold
First break apart your Styrofoam into no more then 1 inch chunks. and fill the bottom of the box with this. Then fill a bucket or box with enough chunks fill the rest of the mold.
Now once you have the first inch layer of the bottom of the mold filled its time to use the great stuff to bind it all together.