Attach various pieces of 20mm slotted aluminum extrusion together with this 5-hole corner plate.
Measures 60mm long x 60mm wide, 4mm thick. 5.3m drill holes 20mm apart so you can use M4 or M5 screws. You'll want to use 10mm long screws to attach this to 20mm extrusion.
L-Plate for 2020 Aluminum Extrusion (8:50)