Medium size cargo solution for road-bike for commuting Currently I use a touring bike with rear rack and a single panier on my 15km commute. In my panier I carry a full set of clothes including shoes, a small coat and a towel. I want to start using my road bike on my commute because it is a better bike in almost all ways except for the cargo situation.
The panier I use is an ortlieb back-roller classic. It is huge, it is completely waterproof, and I fix it to my bike in one quick motion. I'm looking for something that matches or improves on most of those characteristics but fits on a road bike.
More specifically, I'd like to find a cargo solution for my road bike that:


is at least 15-20 litres.
is waterproof in standard setup (no rain-covers).
does not interfere with my biking.
attaches and detaches with minimal effort.
(bonus points) is aerodynamic.
(bonus points) looks good


I've tried a backpack, but that violates point 3 and (somewhat) 5. Also, it is sweaty and bad for my back. I've been looking into the various bags that attach on top of a rear-rack such as the ortlieb trunk bag and the topeak trunk drybag but they are too small. Topeak has bigger models but those have a strange compartmentalisation, and they are not waterproof (since you can buy separate rain covers).
I'd like to avoid the asymmetry of a single panier, and I'd also like to avoid attaching a full rear-rack to my roadbike. Seatpost-mounted rack is fine though.

Edit (2014-12-01): In the end I decided to go for a backpack, the ortlieb packman pro 2 to be exact. Reasons:


100% waterproof
some bike-friendly features (notably, some ventilation on the back)
Exactly the size I need.
Quicker and better ease of use than all rack-mounted or seatpost-mounted solutions I could find.
I would really recommend re-looking at a backpack.  There are a large variety of packs out there and the drybag styles ones seem to get better every year.  I have found that a backpack is a more dynamic way to carry weight.  You might look for one with better airflow built into the padding section.  I find that if I dress just a little more lightly and have a pack with good airflow, "sweaty back" is a bit less of a problem.

If you can't find a drybag style pack that works for you, you could always look for a base pack that fits all your requirements and then just stuff a uber-light drybag in it.

Along those lines, you might also look at Revelate Designs new holster bag.  Essentially it sits under your seat and provides a secure place to stuff a drysack with ease of on/off for the drysack.  Your personal opinion will dictate whether it meets requirement 6 or not.