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 - - Amazon.com
CHERNE Gripper 6 in. ABS Plastic Mechanical Test Plug 270261 - The Home Depot
OEM waterproof small silicone rubber plastic plug with hole - ETOL
Brackit 100 Pack Masonry Rawl Plugs - 6x30mm (1/4"x1-3/16") - Heavy Duty Plastic Wall Plug Fixings for Brick, Concrete, Stone – Ideal for Building or Hardware - - Amazon.com
Oatey Gripper 3 in. Plastic Mechanical Test Plug 33402 - The Home Depot