Amazon.com: (Pack of 2) 4" OD Round Plastic Cap Plugs | (Multi Gauge - ID 3.56" to 3.74") Heavy Duty - Impact Resistant Tough Design || Fence Post Slide Inserts | Fitness
Amazon.com: 3" Square Tubing Heavy Duty Plastic Plug End Cap Insert 3x3 8-14 Tube Pipe Post (15) : Industrial & Scientific