The cover structure comprises a circular plate, a number of fastener components 5 and a number of engraved holes 41 facing outward on the surface the said circular plate (when installed) with reinforcement edges 42 extending towards the interior side, inclined towards the center of each hole and formed at their final extents with end holes smaller than the engraved holes. The fastener components 5 are positioned on the surface of the said circular plate facing inward during assembly and a retainer mount 51 is formed at the end. Each component 5 is positioned in between the edge reinforcements 42 and the end holes of two pairs of the engraved holes 41, and the distance from each component to the center of the circular plate is less than that for the maximum outer edge dimension around the interior width of each engraved hole. When stacked, the fastener components 5 of an upper cover structure are in alignment with the engraved holes 41 of a lower cover structure, enabling the components to fit into them.
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