Dacron Fibre is used as a comfort layer that is wrapped around the foam. The polyester fibre makes it easier to stuff the cushion into the cover. It gives the cushion a more rounded look on the edges and it also fills the cushion properly and keeps the fabric from wrinkling.
You can order the Dacron Fibre separately in a number of different thicknesses and widths and wrap the cushions yourself. The fibre compresses easily and you do not need to make an allowance for the thickness of the fibre when you are calculating the thickness of the foam.
You can use a spray adhesive from your local hardware store to bond the fibre to the foam.