An occluder device is provided for occluding a cardiovascular defect or a gap between a medical device and adjacent body tissue, the device including a compliant balloon defining a fluid-tight balloon chamber and a balloon channel forming a longitudinal passage from a proximal to a distal side of the balloon, the balloon including a fluid port for filling a fluid into the balloon chamber. A tip and a base are coupled to the distal and the proximal sides of the balloon, respectively. At least one connecting strut is attached to the tip and to the base. An elongate actuator is disposed longitudinally slidable in the balloon channel and connected to the tip, and longitudinally slidable with respect to the base so as to set a distance between the tip and the base. A lock is configured to maintain the distance between the tip and the base. Other embodiments are also described.