Wolf: I contacted O.M.M. to see if their Sherpa rack is available in silver.

Pra: The Tubus adapter looks good, but the two-hole-per-side mounting system (vs one-per-side) would mean me buying one of their racks and a seat-tube mount.
I think that in the name of economy, I might go with this RockBros gizmo rated at 25kg: