A hermetically sealed flat container for containing loose materials may include: first and second flexible sheet elements, having polygonal shape, which are coupled together by welds formed along a perimeter of their polygonal shape, to define a volume for containing the materials. The perimeter may define a plurality of edges, one edge forming a bearing edge for the container to rest upon a base. The bearing edge may progressively reduce its transverse dimensions toward one end such that, when the bearing edge rests on the base, a weight-force component of the materials applied to the bearing edge causes a reaction that causes the bearing edge to retract into the volume and imparts vertical stability to the container. The container further comprises a strip of stiffening material attached to the first and second flexible sheet elements, the strip spaced from the bearing edge except for end portions of the strip.