I’m assembling a small free-standing aluminium pergola and I need to raise the legs by just 20mm.

Each leg has a 160mm square, 10mm high detachable plate (pictured) with 3 holes for the bolts. Ideally I’d put some sort of 20mm square riser underneath and line up with equivalent drilled bolt holes.
What could I use that would support up to ~50kg per leg?