I'm new to the idea of going long enough distances that I would actually need or use a tank bag, but I was looking at the Sargent Super Bag, and it is quite impressive! Best thing that I can see from their webpage is that it can be modified to many different sizes to fit your needs. From a daily commute to a cross country trip, I suppose. Here's the page....
www.sargentcycle.com/superbag.htm