Brace yourself! You’ll definitely want to pick up a couple of these corner brace supports when working on a project with 20×20 extrusion. They’re nice and solid, made of cast aluminum so they are stronger than extruded corner supports. Two holes, one in each corner allow you to easily connect two pieces at a 90 degree angle.
Comes as just one piece, you’ll also need T-nuts and 8mm long screws (10mm are too long!)