Brace yourself! You’ll definitely want to pick up a couple of these double corner brace supports when working on a project with 20×20 extrusion. They’re nice and solid, made of cast Aluminium so they are stronger than extruded corner supports. Four holes, two 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 10mm long screws (8mm will also work)