You could put some old inner tube underneath to stop the paint scuffing? Works wonders on a front rack that I’ve done the same way with no clips. Rubber helps with the grip so it doesn’t slip, tiny bit of vibration resistance for the bolts and protects the paint.
I guess I can say goodbye to the paintwork under the Tubus clamps. At least they look like they won't be moving much.
And that AR-1 should be exactly what I need... are racks like that simply not made any more?