Switch to Edge subobject mode. For each leg select the three rings of edges that define the leg's shape. Chamfer the selection using an amount of 0.25 - 1.0 units.
Now select the edges that define the shape of the back of the chair and chamfer them as well with 0.25 - 1.0 units.
Finally select the edges that define the shape of the original box and chamfer these as well 2.0 units. Add a MeshSmooth modifier with 3 iterations and your plastic chair model is finished.
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