The problem with fixing your laptop to the roof, assuming you do overcome the mounting problem, is heat dissipation. Make sure all air vents are clear, mount it with a gap between it and the roof if possible. Surely some way of mounting it to the back of a seat is the best option. It is possible to run a aux screen from a laptop, cables are readily available but an aux screen (LCD, most probably) would probably cost more than the laptop. What about buying one of the new portable DVD players? Like the sort Panosonic and Goodmans sell, about