the bladder its length is larger by a projecting portion than the double width of the bladder 20 the projecting portion causes the end portions of the inner layer 28 of the folded pre cut part to overlap for example overlap over 5 to 20 cm preferably 12 to 18 cm and in particular 14 cm in the case of the inner layer 28 according to fig1 this overlapping portion lies on the underside of the bladder 20 it is invisible in fig1 an outer layer 30 which also closely envelops the bladder 20 and also the inner layer 28 and extends over it is now wrapped around the inner layer 28 a pre cut part having a rectangular shape is also used for the outer layer 30 it has a width corresponding to the width of the bladder 20 with an added portion of about 1 to 5 cm for the inner layer 28 and has a length corresponding to the double length of the bladder 20 plus at least 10 cm preferably up to 25 cm thus a fold over portion 32 with a length of several centimeters is obtained preferably a fold over portion 32 of about 14 cm which later leads to an overlap of the two end portions of the pre cut part on the top side of the bladder 20 the overlap of the outer layer 30 is on the other.