An endovascular stent system (10) includes first and second generally tubular stents members (20, 22), which are shaped so as to define first and second interface sections (40, 42), respectively, which are securely coupleable to each other. The first interface section (40) is shaped so as to define an opening (44). The second interface section (42) is shaped so as to define a neck portion (50). When the first and second stent members (20, 22) are coupled together, the neck portion (50) is at least in part defined by: (a) first ones (72) of structural elements (70), positioned at respective first circumferential locations, which are configured to apply, to the opening (44), distally- and radially-outwardly-directed forces, without applying any proximally-directed forces, and (b) separate from the first structural elements (72), second ones (74) of the structural elements (70), positioned at respective second circumferential locations different from the first circumferential locations, which are configured to apply, to the opening (44), a proximally- and radially-outwardly-directed forces, without applying any distally-directed forces.