A mushroom type hook structure comprising: a stem portion; a substantially planar head portion formed at a top end of the stem portion, the head portion comprising: two protrusions each protruding outwardly from a respective one of two opposite sides of the head portion in a first direction along a portion of a length of the side; an extended portion extending outwardly from a side of the head portion in a second direction that is perpendicular to the first direction along a substantial length of the side, wherein a free end of the extended portion curves downward slightly as a hook; and four connection portions formed at locations where one lateral side of each of the two protrusions adjacent to the extended portion meets the extended portion and the other lateral side of each of the two protrusions meets an adjacent side of the head portion in the second direction characterized in that the mushroom type hook structure is injection molded and the stem portion is of substantially tetragonal shape and includes two straight vertical side walls in a first direction and a straight vertical side wall and a slanted vertical side wall in a second direction, wherein the first direction and the second direction are perpendicular with each other. a stem portion; a substantially planar head portion formed at a top end of the stem portion, the head portion comprising: two protrusions each protruding outwardly from a respective one of two opposite sides of the head portion in a first direction along a portion of a length of the side; an extended portion extending outwardly from a side of the head portion in a second direction that is perpendicular to the first direction along a substantial length of the side, wherein a free end of the extended portion curves downward slightly as a hook; and four connection portions formed at locations where one lateral side of each of the two protrusions adjacent to the extended portion meets the extended portion and the other lateral side of each of the two protrusions meets an adjacent side of the head portion in the second direction characterized in that the mushroom type hook structure is injection molded and the stem portion is of substantially tetragonal shape and includes two straight vertical side walls in a first direction and a straight vertical side wall and a slanted vertical side wall in a second direction, wherein the first direction and the second direction are perpendicular with each other. two protrusions each protruding outwardly from a respective one of two opposite sides of the head portion in a first direction along a portion of a length of the side; an extended portion extending outwardly from a side of the head portion in a second direction that is perpendicular to the first direction along a substantial length of the side, wherein a free end of the extended portion curves downward slightly as a hook; and four connection portions formed at locations where one lateral side of each of the two protrusions adjacent to the extended portion meets the extended portion and the other lateral side of each of the two protrusions meets an adjacent side of the head portion in the second direction characterized in that the mushroom type hook structure is injection molded and the stem portion is of substantially tetragonal shape and includes two straight vertical side walls in a first direction and a straight vertical side wall and a slanted vertical side wall in a second direction, wherein the first direction and the second direction are perpendicular with each other.