Abstract: A rectangular sheet is configured so that both side portions of the main portion sandwiching a center line in the widthwise direction of the sheet main portion are each folded in half along a folding line extending in a longitudinal direction provided at position bisecting a width of each of both side portions to thereby form a vertically long sheet, the vertically long sheet is folded in half at an intermediate portion in the longitudinal direction, the sheet folded in half is further folded in half at an intermediate portion thereof in the opposite direction so as to fully overlap with each other, both vertical end sections of the overlapped portions are folded to form a triangular shape with the peak located at the center portion in the widthwise direction to thereby form an extraction finger grip portion.