You may try short metal zippers by taking an awl (icepick) and punching holes in the sides. that way they may be added to the crochet. Another things is to us buttons with larger holes and thread the hair tie through the button(do not cut the tie in too), thread both sides thru the button). Then add the hair tie to the mat by crocheting the two sides of the hair tie into your design. Add an additional tie close by to put over the button and be able to take it off again.