How about keeping the back rack on and getting one of those bags? http://www.topeak.com/products/Bags/MTXTrunkBagEXP
You may want to have a few more things with you, if the weather sucks. Like extra pair of bike shorts, so you don't ride in wet ones.
And you could keep the extra water bottle in the bag, if you don't have a second mount.

If you decide you want to use the front rack, you can borrow mine, B.