I used a pattern once that called for headliner foam. It is 1/8 " maybe a little less, foam with either one or both sides covered with something that looks a little like nylon tricot. It gave great body to the bag, it stood up fine and wasn't as heavy as the bag I used warm and natural cotton batting for. That is the only thing I use now, at least for bags that are supposed to stand up. I made one that was a slouch type bag with the interfacing recommended by the pattern, but only because it was supposed to collapse. You can get the headliner foam at Joann's Fabric (I usually us a coupon). I have washed my bags on gentle and dried on low in the dryer with no problems.