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Just a screw, a wing nut and a couple of washers.
The trick, if any, is that the actual rotating square is made of two layers of plywood. The one on top has a 15 mm hole and the 2nd has a 4 mm hole for the screw. The screw head+the washer have to protrude less than 3 mm (or the thickness of the top plank).
You don't need to tighten the wing nut at all since the hole is more or less in the mass center of the reader and velvet makes enough friction to keep the arrangement static. But you don't need the velvet. Naked plywood makes good friction too.
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