CHERNE Gripper 6 in. ABS Plastic Mechanical Test Plug 270261 - The Home Depot
Amazon.com: Prescott Plastics 1" Inch (25.4 mm) Round Plastic Hole Plugs, Inserts, Black End Caps for Metal Tubing, Hardware Plug, Fences, Glide Protection from Chair Legs and Furniture - Pack of 4 :