any suitable way such as by adhesive or welding it is evident that for all embodiments the different bonds represent full circumferential attachment sections as otherwise leakage between the different structures may occur 0031 fig2 a illustrates an embodiment comprising a flange 100 and a base plate 200 the flange 100 comprises an inner circumferential portion 100 with an inner edge 103 and an outer circumferential portion 102 with an outer free edge 104 in accordance with the object of the invention the outer portion is formed with a reduced thickness to provide a higher degree of flexibility as compared with the inner portion the transition area 106 between the two portions will establish a u201c virtual u201d hinge function between the two portions the innermost part 105 of the inner portion in the vicinity of the inner edge is formed with a reduced thickness this allowing a good transfer of heat in a heat welding process thus assuring a narrow bond 205 with reduced risk of heat damage to the flange and or base plate the base plate 200 comprises an inner edge 201 an outer edge 202 a lower adhesive surface 204 and an upper surface 206 on which the flange is attached at 205 corresponding to the inner edge portion of the flange the flange being attached at a distance from the inner edge of the base plate although the flange is shown attached to the base plate only at the inner edge portion of the flange the inner portion of the flange may be attached to the base plate to.