It needs to make a polished groove or smooth surface-divot. Definitely use colored or black plastic, to promote surface-absorption. The real key is to eliminate jaggies. Either your steppers need micron resolution or better, or the laser-sweeping needs adjustable radius (smooth curves, but stepped depth.)

I tell people, if you could play the hologram on a record turntable, the scratches must be silent. No micro-wiggles. The must behave as long, curved mirrors.