A modular filter for ventilation air vents comprises a filtering fabric roll (1) housed within a casing provided with anchoring means (13) engaging the surface (S) of the container which has the air intake port (B) to be filtered; the casing has a slot (9) for extracting the filtering fabric (T), and has a casing (4) comprised of a shaped wall (5) enclosed at the ends by covers (10, 40, 44, 51) or end supports (7, 47) of this wall having a shape corresponding to the shaped wall (5) and maintaining a longitudinal slot (9) at the front edges of the shaped wall to allow the passage for the extraction of the front (F) of the filtering fabric (T) unwound from the filtering fabric roll (1); at least one screw stay rod (8, 54) is placed between two covers (51) or end supports (7, 47) of the shaped wall and is tightened to define the proper distance between the two end supports or covers (51), by tightening the end supports (7, 47) or covers (51) to the shaped wall (5), one at each end, to close the access to the casing, and containing the filtering fabric roll (1) floating and free to rotate when the filtering fabric is dragged from the outside of the modular filter.
Various methods for producing the shaped wall (5) and finalizing the casing (4) of the modular filter are indicated.