A unitary flexible sheet is folded to defines a panel, and first and second tabs. Each of the tabs has an inner layer and an outer layer. Commissural portions of prosthetic leaflets are sandwiched between inner layers of the first and second tabs. The tabs protrude from a first side of the panel, and are shaped such that progressively downstream parts of each tab protrude progressively further away from the panel portion. Other embodiments are also described.