Originally Posted by Big Lebowski

I have the same Topeak Explorer on both of my bikes. FWIW, I have the MTX EXP bag. Easily carries my laptop on one side, a change of clothes and my lunch on the other. Plus, there's a ton of space on top for wallet, first-aid kit, glasses, and all the little stuff. It slides right on and snaps secure. Nice shoulder strap too.

It isn't waterproof, so you'll have to spray it with something. I used the Kiwi stuff. Works great.