Attach various pieces of 20mm slotted aluminum extrusion together with this 7-hole cross joining plate.
Measures 100mm 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.
Cross-Plate for 2020 Aluminum Extrusion (8:50)