Axiom seems to make the strongest one I could find. The Journey seat post holds 15KG/33lbs. I'm going to add one of their seat post collars to the lower part of my seat post, (that part of the seat post that sticks out below the frame), and a couple of braces going to the center rack support. That should allow for a lot more weight. And, it will still allow seat height adjustment and deter seat/rack theft.
