Abstract: The present invention includes: a base section configured such that, taking the unit edge length D1 to be 1 and a cube expressed as 1×1×1 to be one unit size, the base section has a parallelepiped shape in which one or more 1-unit sizes are conjoined, and is composed of an open, hollow face taken to be the bottom, four lateral walls 3B, and a roof 3A; and cylindrical projections having a cylindrical shape and provided on the outer face of one or more of the roof 3A and the lateral walls 3B, with a diameter D2 being equal to or greater than 0.4 but less than 0.6 with respect to the unit edge length D1, and a height being (1?diameter)×(0.4 to 0.5 inclusive).