Plating on 3D printed ABS resins?
I have seen printed horns coated with the spray-on conductive paint (e.g. MG Chemicals 843AR) whose gain is only a dB below a metal one, but it was a one-off and I never saw any follow-up. That said, it might be enough for the initial strike plate needed.
As far as smoothness and surface sealing, you should look up what people are doing to ABS with acetone vapor. I've tried it a bit and can attest that, by paying careful attention to temperature and duration, you can get quite nice finishes.