In this paper, we present BaLuna, a prototype of an externally actuated midair display for indoor use in a room-scale workspace. This system is the first battery-less midair display with a one-meter-cubic workspace. The system projects an image onto a balloon screen whose position is controlled by airborne ultrasound phased array (AUPA) devices. Users can naturally manipulate the screen position by dragging and dropping the screen directly with their hands. We adapted feedback-based acoustic manipulation technology that enables sparsely distributed AUPA devices to control the screen position. This is combined with a depth-image-based tracking and a three-dimensionally calibrated projector.