The Salsa Down Under racks need double mid-fork attachment points to provide rotational stiffness when loaded. That is, when loaded and the wheel is turned quickly, there is a tendency for bags mounted on low rider racks to rotate into the wheel spokes.
Low rider rack makers use one of two schemes to provide rotational stiffness, double mid-fork attachment points or a hoop over the wheel attaching the two racks together. P-clamps can do a passable job of supporting a load, but I'm skeptical of their ability to resist twisting rotational forces. At least, it will be inexpensive to try them.
If you eventually decide to replace the racks with a hoop style, I can highly recommend the Tubus Tara. I recently moved a Tara from my old Fuji Touring frame to my new Soma Saga frame. The load bars on the Tara racks are dead level when installed in the middle (of three) upper attachment holes. (Unlevel racks are my pet peeve. Indicates shoddy work.) It's like the Saga fork was made with Tara racks in mind. I've put over 2000 miles on the Tara racks loaded so far and they kick the snot out of the two other racks I've tried (Blackburn and Surly Nice Rack) in terms of weight, stability, and load carrying ability.