Abstract: The bottom (1) and the cover (2) are made from a rigid composite material constituted by a sheet of PVC coated with a layer of polyethylene on which is applied a film of polyethylene of peelable quality. The bottom (1) has cavities (4). Peripheral flanges (3, 3') includes ears (6, 6') and are welded along a strip (7) having weakened regions (8). The cover is peelable.