I would stabilise the fabric with a light iron-on interfacing first. If it’s a very fine or stretchy jersey, you can use another layer of interfacing, if needed. To help keep the bag in shape, interline it with a cotton backed foam (I usually sew it to the outer fabric). Annie’s Soft and Stable, Pellon FF78F1 or Vilene Style-Vil sew-in foam batting are suitable.