These heavy duty mounting plates are perfect for any project requiring connection strength.
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, Clear Anodized Aluminum
so you can use M5 screws. You'll want to use 10mm long screws to attach this to 20mm extrusion.