craigslist is your friend... you can probably find a good used road bike with what you are looking for, and to boot, it gives you a 'rain bike' when you don't want to take the allez. Having said that, a colleague of mine has a waterproof bag--sort of like one you would take on a canoe--with the roll-type closure. It has foam with lots of furrows in it that keep the bag from being totally against your back. He rides with it on his racing bike year round and doesn't complain about a sweaty back.
The important thing is that you enjoy riding the bike--whichever it is-- because if you don't like the bike you won't ride it, and then you have one more excuse for why you drove in today instead of riding your bike...

train safe