We love this handlebar bag by Ortlieb. It is large, it is water proof, and it mounts to most handlebars with its quick-release system. Classic ortlieb stuff here -- simple and affordable.
Handle bar bag with lockable magnetic lid closure, including lockable mounting set. For handle bars with a diameter up to 31.8mm, PU-coated Cordura, 2 outer mesh pockets with zip, inner pocket, internal dividers, shoulder strap, reflector. Optional accessories: map case. Mobile Guide and camera insert.