The box assembly comprises a casing (1) comprised of first and second elongated members (2, 3) defining an encircling wall and having respective mutually coupled adjacent first longitudinal edges (2a, 3a) and respective mutually separated second longitudinal edges (2b, 3b) defining a longitudinal opening (4) therebetween, and two closing plates (5) fixed to ends of said first and second elongated members (2) by respective first and second fixing elements (6, 7). Said first longitudinal edges (2a, 3a) of the first and second elongated members (2, 3) comprise respective mutually coupling configurations adapted to allow relative movement of the first elongated member (2) in relation to the second elongated member (3) enough to provide an access to the inside of the casing (1) when said first fixing elements (6) are removed, while said second fixing elements (7) remain coupled and keeping the second elongated member (3) fixed to the closing plates (5).