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I used a small spanner (wrench) that came with my kid's electric scooter. It is made out of stamped steel and cut out in wrench shape. I suppose an inexpensive bicycle tool would work too. From memory I believe the opening is 10mm.
Here are my links with scooter spanner and wrench. The nuts are 10mm but the inside flat surfaces may be 8mm. The spanner is 10mm on one side and 8mm on the other.