Car Inductive Charger Module?
The guys over at seeedstudio sell a ready-made inductive transmitter/receiver pair, designed specifically for charging batteries. Input voltage is 12V, so it would be an excellent fit for automotive applications. I have no experience with it, but at $\$$9.50 it might be worth a shot. 

Both the transmitter and the receiver should fit in a flat 50x60mm housing, so i suggest you design your product on a similar footprint. It is hard to comment on ergonomics without knowing at least the shape of what you are planning to charge, the approximate interior arrangement of the vehicle and whether it's intended to be operated by the driver or the passengers.