The invention comprises a bag folded towards the axis of symmetry thereof via the opposite sides thereof. The edge of the sealed upper end of the bag has a cut-out in the form of a circular arc and two eyelets or securing means. The bag has a lateral opening and an additional layer of the same material with a corresponding opening is adhesively bonded thereon, forming a casing or conduit that completely surrounds the opening. This casing contains a thread, the ends of which, crossed over the opening, are joined to the emerging ends of a pair of removable slides or blocking means positioned on the casing. A rigid part is adhesively bonded horizontally below the latter, said part having fixing notches at the ends, in which the free ends of two elastic threads operationally engage, whilst the respective opposite ends thereof are fixed to each of the removable blocking means.
