SW-Motech Pro Tailbag gets my vote as a stylish, expandable, high quality and functional tailbag. This Pro version (link below) is the new improved one over my outgoing model SW-Motech tailbag. I have had my tailbag for many years now and it is still a great bag that I use virtually all of the time on three different bikes. It is concave on the underside and is designed to sit on a seat and not a rack. They come with a waterproof inner back rather than the rain covers you get on the tank bags.
Cons: they are not cheap, not good for flat surfaces such as a rack
Pros: Quality feel, has server me well, zips have eyelets to use their optional security lock and cable, bag still in good condition.
https://sw-motech.com/en/products/luggage/tail+bags/PRO+tail+bags/4052572198070.htmThe picture is with it on my rear rack although it is not really designed for that with its slightly concaved base.
