The Wald 585 is made to go on top of the rack, and should carry anything your rack can handle. Honestly, though, it just seems like a $45 alternative to u-bolting a stolen milk crate onto the rack. My Tranz-X rack says it's good for 25kg, but I (~90kg) can sit on it with no wobble.

Search on here for "homemade panniers" and you should find plenty of ideas to get the CG lower than a rack-top solution.