-$100 bill (though I suspect this will work with other bill denominations as well as other proportion rectangles)
Fold in bill in half again and then unfold.
Fold both sides in to meet the middle crease then unfold.
Fold the top and bottom in to meet the middle crease.
Fold both the top and bottom from the middle over the folded in corners.
Open the box and fill with your less valuable change.