A system for mounting a secondary panel within a window frame of an existing window. The system includes a rigid panel, an elongated deformable bulb, and an elongated carrier. The bulb has a resilient portion, a base section, an extension extending from the base section, and a crosspiece coupled to a distal end of the extension. The crosspiece includes a pair of shoulders at opposite ends of the crosspiece. Each shoulder protrudes laterally beyond the extension. The elongated carrier has a receiving slot opposite a panel gap configured to receive an edge of the panel. The receiving slot has a neck laterally narrower than an interior cavity of the receiving slot. The receiving slot is configured to securely receive the crosspiece of the bulb and to confine the shoulders of the crosspiece.