Buttonhole Elastic
It is an elastic product that has buttonholes woven into the elastic. The buttonholes allow you to adjust the fit of the elastic but do require an opening in an elastic casing for accessibility to the elastic and a button outside the casing to hold the elastic.
Don't be afraid to err on the side of extra length. Even if it is longer than you need at first, it's very easy to trim the buttonhole elastic later. Use a safety pin to feed the elastic into one buttonhole, through the casing all around the back of the outfit, and then out the buttonhole on the other side.