From t-shirt to small bag. Cute, but I wouldn't use a zipper as a strap...and I'd sew the gathers with embroidery floss or upholstery thread rather than using rubber bands.
Pillow case bag- trim 1/4" off closed end, cut in half on the diagonal. Turn fabric of each of the 4 diagonal cut edges over by 1/4" twice. Pin/hem. Place one pillowcase half inside the other, lining up bottom edges. Pin & sew at front where fabric overlaps. Repeat on other side. Turn right side out. Tie top ends into a knot.