I would not mount a laptop to the roof of any vehicle. The laptop will not last very long... not from the vibration or even from falling. Laptops are designed to dissipate heat from bottom to top. By turning the laptop upside down the heat will now be going to the bottom of the laptop, which is now your top. This will burn out the cpu, memory, video and eventually the hard drive. It might last a few weeks or a few months, but it will not last very long.
Heat kills computers and putting one upside down will certainly kill a laptop FAST.