Take an old sweatshirt or jumper and simply sew both arms to the body, joining the cuffs together. Sew up the base of the jumper. Stuff the centre with wadding then sew up to close the centre area (following the arm hole line to complete the circle). Now stuff the arm holes and sew up the neck opening. These fit perfectly into the right size resin base and are machine washable too.
This was the bed I made for Pitilín out a polar fleece.