Features. If you punch out too many knockouts on the under gravel filter, it will not function correctly. Knockout plugs snap into the holes to cover them, restoring an otherwise inoperative filter plate. Fits lees original and economy under gravel filters. Specifications. Weight 0.17 lbs. These high-grade Prescott Plastics black ribbed plugs work great for preventing scuffs on wood floor or as protection glides on chair tips, bench feet, table legs, bar stool foot sliders, chair leg protectors, patio furniture inserts and much more. Our plastic end caps and inserts are designed to hold up in all harsh weather conditions and high traffic areas. They are also designed to hold up against impact abuse and harsh weather conditions.