Stayput White Attachment Knobs are used with Shock Cords for Canvas which are Sold Separately Use our bungee attachment knobs anywhere you need a piece of cloth held down securely, from mooring tonneau boat covers to curtains! The Shock Cord Cover Clip is installed by making a small hole in the fabric and snapping its self-locking sections together. Order Lacing Knob to complete the assembly. Acceptable size shock cord: 3/16" to 1/4" The height of the knob is 9/16" The OD is 11/16" (base) Recommended screw size is #10 x 1.5" Flat Head.