Abstract: A high-stability composable case of the type comprising a thermoformed element made of plastic material consisting of a central panel (12) having a quadrilateral shape, the sides of which are connected, by means of fold-inviting lines (17, 18), to four sides of side panels (13, 14, 15, 16), which may be folded, in a box-like manner, on the central panel (12), there being provided corner elements for forming and blocking (25) between the central panel and the side panels, where the side panels are equipped with flap-like edges (20, 21) which protrude outwards with respect to the side panels (13, 14, 15, 16) and which can be set alongside one another in a closed box-like position of the case to be inserted two by two within a slit (37) of each corner element for forming and blocking (25), in which the flap-like edges (20, 21) provide longitudinal and transverse elements for engagement and connection (23, 27; 29, 30; 31, 32) designed to bring about a stable reciprocal positioning between the flap-like edges (20,