The hollow body includes the storage chamber for electrical devices such as lamps, sockets and clamps, held to a shell. There is a cylindrical rear part closed by a front part fastened to the concrete. The front part (20) has a projection whose opening is at first closed by a number of walls that can be removed later. The walls have different diameters so that by selective removal, different diameters of opening can be produced. The walls are circular rings joined by bridges. The group of rings is joined at one end with the circular base (17) of the front part and at the other end with the middle of the projection.
