First, I drew some shapes on cardboard; one is shown below. I added an arrow to show you which way the corrugations run, as this will be important later. You're essentially making a frame, so you'll need an inner and an outer shape. Use a craft knife (adults, or fearless and skilled older kids, only) to cut out the shape.
Get a long, blunt yarn needle, and thread it with a few feet of cord. Insert one end of the cord through a corner channel, and out the neighboring one. Tie the end to itself to anchor the cord, then slip the excess cord into the channel.
Here's how it looks, knotty-side up, all nobbly and wooly: