To suitably change a depth or a shape by providing a large opening part in the central part of a rectangular plate, providing a sail opening part in the position suitably apart from it, and providing a plurality of bendable weak parts in parallel with the left and right edges of the large opening part at a suitable interval.

A large opening part 3 is provided in the central part of a rectangular plate 2 comprising a metal plate such as an iron plate or a synthetic resin plate, and small opening parts 5, 5' are provided in the position suitably apart from the left and right edges 4, 4' of the opening part 3. A plurality of bendable weak parts 6 are provided in parallel with the left and right edges 4, 4' of the large opening part 3 at a suitable interval from the left and right edges 4, 4' of the large opening part 3. Therefore, dimensions such as a depth or a shape can be suitably changed in conformity with the conditions or the like of a fitted place.