Somebody up thread posted the 2 Wheel Gear pannier backpack conversion bag. That is what I use for my laptop, and it is an excellent product all around. Pricey but it makes life easier.

It hooks onto my rack securely and locks tightly, and is of high quality in terms of zippers and materials etc. Off topic - Don't trust the frame mounts for the Kryptonite lock. It failed the other day and the lock dropped onto my ankle. Not even a month old..I have to figure out a solution.