Quite a few racks come with the optional hardware to attach directly to the seatstays, which in most cases are the aforementionde p-clips. Some have two metal plates which would clamp around the stays.
Or maybe an Old Man Mountain might do the trick if you have cantilevers
This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.