Apparatus, comprising: a unitary flexible sheet, folded to define: a panel, defining a plane, and having a first side facing in a first direction away from the plane, and a second side that is opposite the first side; a first tab, disposed on the first side of the panel, and protruding in the first direction away from the panel; and a second tab, disposed on the first side of the panel, and protruding in the first direction away from the panel; a first prosthetic leaflet having a first-leaflet commissural portion disposed between the first tab and the second tab; a second prosthetic leaflet having a second-leaflet commissural portion disposed between the first tab and the second tab; wherein: at the first tab, the sheet is folded to define a first cushion at a downstream edge of the first tab, the first cushion being configured and positioned to inhibit deflection of a downstream portion of the first prosthetic leaflet away from the second prosthetic leaflet and toward the plane, at the second tab, the sheet is folded to define a second cushion at a downstream edge of the second tab, the second cushion being configured and positioned to inhibit deflection of a downstream portion of the second prosthetic leaflet away from the first prosthetic leaflet and toward the plane, the sheet is folded such that the first tab has a first-tab outer layer and a first-tab inner layer, and the second tab has a second-tab outer layer and a second-tab inner layer, the first-leaflet commissural portion is disposed between the first-tab inner layer and the second-tab inner layer, and the second-leaflet commissural portion is disposed between the first-tab inner layer and the second-tab inner layer, characterized in that: the first tab defines a downstream opening between the first-tab inner layer and the first-tab outer layer, and the second tab defines a downstream opening between the second-tab inner layer and the second-tab outer layer. a unitary flexible sheet, folded to define: a panel, defining a plane, and having a first side facing in a first direction away from the plane, and a second side that is opposite the first side; a first tab, disposed on the first side of the panel, and protruding in the first direction away from the panel; and a second tab, disposed on the first side of the panel, and protruding in the first direction away from the panel; a panel, defining a plane, and having a first side facing in a first direction away from the plane, and a second side that is opposite the first side; a first tab, disposed on the first side of the panel, and protruding in the first direction away from the panel; and a second tab, disposed on the first side of the panel, and protruding in the first direction away from the panel; a first prosthetic leaflet having a first-leaflet commissural portion disposed between the first tab and the second tab; a second prosthetic leaflet having a second-leaflet commissural portion disposed between the first tab and the second tab; at the first tab, the sheet is folded to define a first cushion at a downstream edge of the first tab, the first cushion being configured and positioned to inhibit deflection of a downstream portion of the first prosthetic leaflet away from the second prosthetic leaflet and toward the plane, at the second tab, the sheet is folded to define a second cushion at a downstream edge of the second tab, the second cushion being configured and positioned to inhibit deflection of a downstream portion of the second prosthetic leaflet away from the first prosthetic leaflet and toward the plane, the sheet is folded such that the first tab has a first-tab outer layer and a first-tab inner layer, and the second tab has a second-tab outer layer and a second-tab inner layer, the first-leaflet commissural portion is disposed between the first-tab inner layer and the second-tab inner layer, and the second-leaflet commissural portion is disposed between the first-tab inner layer and the second-tab inner layer, characterized in that: the first tab defines a downstream opening between the first-tab inner layer and the first-tab outer layer, and the second tab defines a downstream opening between the second-tab inner layer and the second-tab outer layer.