Check out -Cortech SportTail Bag, Well made,many options, expandable, I'm on my fifth year with this very bag ! Good Luck !
I am past 5 years with mine and I just came home from grocery shoppin with a 12 pak of pepsi in it, using the
expanded option. On longer trips it rides on top of a duffel across the rear seat,using the attached bungees.
I use a cortech tankbag every day, and my Sport tailbag most days.