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, rounded portion and a base section with at least one tongue extending from the base section. The carrier is configured to receive at least a portion of an edge of the panel within a panel gap and to receive, between opposing resilient prongs, the tongue of the bulb. The resilient prongs are configured to diverge to allow a distal end of the tongue to pass between the resilient prongs. Also disclosed is a system for mounting a flexible sheet within a window frame of an existing window.