Replace tension bar. Press seam, trim off excess fabric, and turn tube right sides out. Insert elastic through the tube until it emerges from the other end. Tie the two ends in a knot. Slipstitch together, being careful not to catch the elastic in stitching.
Plastic headbands are easy to wrap with a bias-cut strip of fabric.
Spread a thin layer of glue on outside of headband. Trim the foam to fit edges of headband. Cut a bias strip of fabric at least twice the width of the headband and about two inches longer. Spread a thin layer of glue on underside of headband. Embellish headband with buttons, ribbons, etc.