Cut out 2 rectangles from fabric. Pin together, right-sides facing, along long sides; sew with a 1/2-inch seam allowance. Turn right-side out; press with iron.
For channels, double-hem short sides: Fold over 1/2 inch, then 1 inch, and pin in place; edge-stitch ⅛ inch from inner edge. Fold bag in half, outside fabric facing, so hems meet.
Stitch long sides with a ¼-inch seam allowance, stopping 1 1/2 inches from hems.
For ties, use tape or cord, or make your own: Cut 2 strips of fabric, each 2 inches wide by twice the length of top of bag plus 3 inches. Fold edges in lengthwise so they meet in the middle; press. Fold strip in half lengthwise; press.
Make an ⅛-inch edge stitch along open edge. Attach a safety pin to one tie; thread through both channels, working left to right. Do the reverse with other tie, threading from right to left. Knot ties; turn bag right-side out.