An assembly of a first element, such as a fan shroud, with a second element, such as an air guide, realized by assembling an elongate thin lip belonging to the second element in an elongate narrow slit formed by the three sides of an elongate planar plate attached in parallel to a planar wall of the first element. The elongate thin lip includes a concave edge and includes, at either side thereof, extended tips, and the elongate slit includes, at either side thereof, areas in which the thickness of the slit is increased, configured to engage with the extended tips of the lip so that the insertion of the lip into the slit begins by inserting the tips into the areas and then continues with gradual insertion of the rest of the concave edge of the lip as the lip is being advanced.