I am using a Mountek nGroove Snap 3 Magnetic mount, it goes in the cd slot. I use the large metal plate inside my case between the layers. I am using this case:
i-Blason Amazon Fire Phone Case- Armadillo Series 2 Layer [Ultra Slim]
Search both of those at Amazon to see pics.
Heavy phone it moves around a bit, but hasn't fallen off. I use the same setup with a Galaxy Note 2 with an extended battery which is pretty heavy, and it only fell off once or twice.