What are the key considerations when selecting a gravel bike frame bag with a waterproof zipper, and how do manufacturers balance the need for waterproofing with the requirement for easy access to the contents of the bag, particularly when riding in wet conditions? Are there any specific zipper designs or materials that are more effective at keeping water out, and do these designs compromise the overall functionality of the bag? Additionally, how do frame bag designs with waterproof zippers impact the bikes overall aerodynamics and handling, and are there any notable differences in performance between bags with waterproof zippers and those without?