The Kreiga Universal System (US) bags are really nice and go straight on to the loops already provided on the bottom of the seat. They also make a tank bag adapter for their bags as well. The roll-top dry bag design makes them super waterproof, but decreases usability for the same reason - a zipper top is more convenient and far less weather proof. They come in a number of sizes and can be attached to each other to configure for your use case.
I have a US20, 20 litre bag, that I use for my daily commute. It fits a pair of running shoes + gym shorts/shirt/socks + a light weight thermal top with a little bit of room to spare. I find the proportions of this bag to be the most aesthetically pleasing on the tail of the Supersport. The 10 looks like a peanut, and the 30 looks like a touring bag. Probably the right size for rain pants plus a little bit more, not big enough for a two piece rain suit though - you'd want the US30 for that.
The attached photo is the US20 filled to capacity on the back of my bike.
I have a US20, 20 litre bag, that I use for my daily commute. It fits a pair of running shoes + gym shorts/shirt/socks + a light weight thermal top with a little bit of room to spare. I find the proportions of this bag to be the most aesthetically pleasing on the tail of the Supersport. The 10 looks like a peanut, and the 30 looks like a touring bag. Probably the right size for rain pants plus a little bit more, not big enough for a two piece rain suit though - you'd want the US30 for that.
The attached photo is the US20 filled to capacity on the back of my bike.