What you'll need:
4"-6" of elastic, I used 1/4" braided. It was 5" unstretched.
Light weight interfacing for the top piece
1 13"x3 1/2" strip of fabric for top
1 7"x 1 1/2" strip of fabric for elastic casing
1. Iron your interfacing onto the top piece, and cut to size.
3. Iron the right sides together lengthwise, and sew a 1/4" seam along the long side to form a tube. Turn right side out, and press, with the seam in the middle of the back.
4. Fold your casing piece in half lengthwise, and press. Sew a 1/4" seam down long side, forming a tube. Turn right side out, and press with seam in the middle.
5. Thread elastic through casing, and pin each end into the openings of the top piece.
6. Sew openings closed.
If you want to make it larger, just increase the length of the top piece.