Aluminum Base Plate, 4" x 4", Thickness is 3/16" and Hole size is 3/8".
H21453 40S 80mm x 80mm BASE PLATE W/16mm TAP IN CENTER
Amazon.com: Powder Coat Paint Aluminum Square Base Plate, Post Base Anchor with Installation Pack for Cable Rail Deck Mount Posts (Black) : Tools & Home Improvement
ProX XT-BP3636A 36" Aluminum 8mm Truss Base Plate for F34 F32 F31 Conical Square Truss with Connectors | ProX Live Performance Gear
Amazon.com: Global Truss Base Plate 30X30A 30" x 30" Aluminum Base Plate for F24/F32/F33/F34/F44P : Everything Else