A lower wall (22) is arranged at a distance beneath an upper wall (21). The lower wall (22) is convex from a central region (23) and then at least approximately espouses the shape of the upper wall (21). An overlap region (27) is thus formed. On the opposite side the lower wall (22) is bent upwards parallel to and flush with the longitudinal edge (26) towards the upper wall (21) and follows the latter back to the centre of the plate. A second overlap region (28) is thus formed. This design avoids the formation of a hollow space in the bonding region of the neighbouring metal plate. In addition, it permits the insertion of metal strips running parallel to the longitudinal edges of both walls. It is also possible to fit a spacer extending over the entire central region (23) in the space between the upper (21) and lower (22) walls to reinforce the entire structure.