Adjuster beads from Rings & Things work well with 1mm and 2mm cotton cording and leather cord. These two-hole beads are easy to use:
String two adjuster beads onto one cord end and tie a knot.
Take the other end of your cord and string it through the second hole of both adjuster beads in the opposite direction of the first cord end.
Give yourself one to two inches of slack, and tie a knot. The beads should "grip" the cord and make it possible for the wearer to slide the piece open and closed according to their needs.
Lace Adjusters (Clearance) are similar, but usually don't require knots. Frequently the tension caused by the lacing going through the holes, is enough to keep the adjuster safely in place.