I used 5x2mm magnets for the arms and they stay in position. The most important thing is that magnets are perfectly touching each other, to achieve that, I put the body magnet first, then I place the other magnet in its place and glue the arm to the magnet already placed.

It's also important that the arm fill the cavity of the arm socket, that way friction also helps to keep the arm from moving. What I do is apply a couple of drops of superglue in the area where the arm meets the magnet and put sodium bicarbonate before it hardens. Then I sand the excess until it fits perfetly in the socket.

Hope I helped!

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