Option 1. Ironwood.
The problem was that the 4 wheel brackets needed the wheel recess deepened by 2.5mm, and the brass shape had few clampable surfaces.
So, I tried option 1.
I machined a wooden jig to hold the bracket in the milling machine vice. The wood is Australian desert ironwood, which is unbelievably hard, but would not mark the brass. The pocket was machined to the diameter of the circular base of the bracket, and then sawn in half.
The bracket was clamped in the jig and squeezed tightly in the vice. Then machined with the slot cutter, the required 2.5mm deeper. The workpiece showed NO tendency to move.