I wanted to go Topeak specifically for the quick-release track interface design between their trunk bags and racks. I really love the design.

On my setup with the Problem Solvers seat post clamp, even though the mounts are below the clamping part, they're still slightly above the height of the rack deck. Topeak recommends a slight forward slope of the rack, which I did, and the rack front brackets are bent slightly upward to attach to the seat post clamp.