I have a cheap proposition for you. Buy a backpack, padded and soft. Cut the shoulder straps off or turn them out. Buy 4 strong magnets and put inside of the backpack around the back side, fix them from moving. Voila. Flat and long, many pockets, huge selection of backpacks..
As I said, cheap, for the idea it will be $10 payable immediately.
PS: I never seen a rain suit fit in a tank bag, unless it is a garbage bag with a neck and arms cut out. Probably not a suit, more like a condom.