A system for mounting a panel within an existing window frame. The system includes an elongated fin portion and an elongated carrier. The fin portion has a ridge section, an extension extending from the base end of the ridge section, and a crosspiece coupled to a distal end of the extension. The ridge section tapers from its base end to a deflectable and compressible tip of the ridge section. 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. 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.