Here is a download for my insert pattern. If you head to the store and pick up a pack of microfiber inserts you can whip up a couple for your stash in under an hour. Simply print out the pattern, use it to cut your towels, and serge around the edges. They can also be turned and top stitched if you don't have a serger. To top stitch them, sew your layers together with a straight stitch leaving one end open to turn them. Turn them right side out once sewn together, pin the end closed and top stitch all the way around.
You can use almost anything for an insert--prefolds, old towels, old tshirts or sheets cut up.