For a very simple bag to make which can be made in any size, take a rectangle of pre-quilted fabric, sew a zipper on the short side, leaving about 1/2" from the ends of the zipper to the edge of the fabric, then sew the 1/2" leaving the ends of the zipper free (which will make a tube). Hold the edge of the fabric tube right sides together with the top of the zipper as the starting point and sew from the zipper to the end of the fabric using a 1/4" seam. If you want a strap, you can use cording or bias tape, cut to the length you want and placed at the end of the fabric on the right side before sewing. On the opposite edge, hold the fabric right sides together with the zipper in the center of the tube, then sew across from side to side. You end up with a weird triangular kind of shape.
I don't have a picture of one, but will try to take one and post a picture.