Very neat lamp!
I have made scalloped fittings like those knobs, in two ways.
One by end-mill and rotary-table as Andy suggests: I cut radially rather than by plunging.
The other by drilling a pitch-circle of holes on what would become the final diameter, then turning the rim down to that, thus halving the holes. That was on a lamp bezel, and drilling under-depth gave a neat closure to each scallop, resembling those on the original plastic moulding I was replacing. (A ball-ended end-mill of course would give a hemispherical rather than hemi-conical run-out. )