Quick and easy! They were also free, as I was able to use up some fabric scraps that were just taking up space. Here's how I did it:
Reusable Sandwich Wrap
This is the size for a 1/2-half sandwich.
two squares of fabric, 9" x 9"
two pieces of narrow ribbon, 6" each
Sew the two fabric squares, right sides together, almost all the way around all 4 sides. Leave a small opening for turning right side out.
Insert one of the ribbons into the opening, extending out diagonally from the corner. Sew the ribbon into place, and continue edge-stitching around the entire square.
This top-stitching eliminates the need to slip-stitch the opening closed, and it will also keep the square in nice shape when it is washed.
At the base of the second ribbon, sew a very small button hole. Should look like this.