a support layer;

a top layer disposed on the support layer,

a base, wherein the support layer and the top layer are stacked on the base, and the base comprises a body having a through hole formed therethrough and an insulation wall protruding from the body towards the support layer as well as encircling the through hole,

wherein the top layer includes a polymeric material having a Shore A hardness lower than a Shore hardness of a material of the support layer, and

the top layer and the support layer have at least one vacuum channel formed therethrough, vertically extending from a top surface of the top layer to a bottom surface of the support layer, wherein the through hole is in fluidic communications with the at least one vacuum channel,

wherein a hemicylindrical concavity is formed at an outer side edge of the top layer, the hemicylindrical concavity extends from the top surface of the top layer to an opposite bottom surface of the top layer, the outer side edge connects the top surface of the top layer to the bottom surface of the top layer,

the through hole is formed in the support layer aligned to the hemicylindrical concavity, and

a threaded hole is formed in the body of the base, aligned to the through hole of the support layer.