You can try foil (copper foil grounded to the chassis would be the most effective), but a lot of times the radiation from switch mode power supplies is magnetic instead of electric, and a thin faraday shield does almost nothing to attenuate a magnetic field. I'm assuming you are complaining about the AM band, and standard AM antennas are especially sensitive to magnetic field interference because they are a coil around a ferrite bar.
Sometimes a ferrite toroid RF choke around the 12v feed wires helps too, in case the noise is actually traveling through the 12v supply rather than being radiated over the air.