Kensington Wall/Air Ultra Compact Notebook Adapter (K38067US)
Kensington is the only company that advertises a "travel adapter" without automotive support. Its previous K33197US was the last Auto/AC/Air combo adapter. Now, the only way to get something that works in the car is with an EmPower-to-cigarette adapter (like the one that comes with the Duracell charger).
Otherwise, if you stand by Kensington's products, you're compelled to buy a power inverter instead (and we already know that's not an ideal solution). The EmPower-to-cigarette outlet adapter is quite hard to come by. Frankly, Kensington should just throw that adapter into the mix, as it already includes it with the K38037US power inverter.
While it's true that this charger is the cheapest of the bunch (~$88), it has a few limitations. The USB port is oddly placed off to the side, which complicates cable management if you prefer to keep the adapter in your bag and spool the power cord. Additionally, this adapter is the only one we're testing with limited voltage output. Instead of determining voltage range based on sense resistors inside the connector's tip, it employs a pin-out system.
There are two SIM-like plastic cards that come bundled. In order to achieve a specific voltage, you need to plug one card into the spring-loaded receptor slot. The 16 V card provides a voltage output between 14-17 V, while the 19 V card outputs between 17-21 V. It seems like Kensington is tiering the voltage output as an additional safety mechanism.
At the same time, you could unintentionally create a problem by using the wrong card, as you'd either get too much or too little voltage. In the case of the former, you could cause real damage to your notebook.