Default Best Case for Magnetic Qi Chargers? - Official Nexus Bumper Magnets?

I'm wondering what case might be best for use with the a magnetic Qi charger (like the official nexus one).

For instance, how does the official bumper work? Does it have magnets built into the case? Are there any cases that do or should I just use an actual bumper?

If I can get the connection strong enough I might try to use the charger as a car mount.
John 14:6