When I got my TW, my friend gave me a Bilt medium size tank bag from Cycle Gear. Because it was too big to use on the TW tank, I mounted it backwards on the top of my rear rack. Backwards makes it higher at the back than at the front and the zippers close to the front of the bag, keeping me from riding with the top flopping open and not knowing about it. Since the bag was soft sided, I had to smash it down to keep in in place.
Eventually, I cut the bottoms out of two 2.5 gallon plastic containers <cat litter> angling the sides to match the side profile of the bag, and just jammed them into the bag (fit like a glove), which created TWO hard plastic lined compartments while stiffening the sides of the bag. With the plastic in place the bag now looks and acts like a piece of luggage.
I carry my loose stuff (MP3 player, camera, hat, earphones, air gauge, 3 way spin-tight, cotton gloves, etc,) in the front compartment. Then I discovered that one of the plastic bottoms fit perfectly into a soft "foam-lined style" lunch bag / sack / box (the kind with soft handles) so I now use the back compartment as a cooler. I drop in a frozen ice bag, two bottles of water, a couple of frozen Uncrustables (pre-made PB&J sandwiches) plus two granola bars (the handles on the foam lunch bag fold over to insulate the top of the compartment). I close the cover, strap it down, and off I go for a days afternoon ride. The plastic container liner even protects the bag from any condensation, and that back section doubles my "carrying space" when not in use as a cooler!
The stiffened bag is easier to secure to the rack, and its visible top window is readily available for small or flat items. Once filled and the top zipped closed, I simply use a 2" wide velcro strap over the top that keeps the bag from moving or wiggling very much. Plus the velcro strap makes it a breeze to get in and out of bag. The bag had some wear on it from rubbing on the rack and moving, but once stiffened and strapped in place, no new wear has showed up. Nicest aspect is that the bag looks natural on the bike and doesn't interfere with swinging my leg over the saddle.
I know. More than you wanted to know.