There is no easier way than how you described. However, the cars electrical system can vary wildly from 9-16+ volts depending on conditions, i.e. cranking the engine, high-speed driving, load of other accessories, etc. This may be harmful to your laptop.
I just wired a Kensington DC-DC adaptor to my car's 12V line, using a relay that is connected to the ignition switch, allowing me to control on/off power to the laptop.