This little hack was sent in by [andrew], he built it specifically for the long flight from Australia to the US. The idea is simple, but I wouldn't have considered it given the cost of decent D Cells these days. H put together a 20 D-cell power pack, a regulator to keep it at 24v, and an eighth inch plug to power up his old ibook.
Update: I goofed and left out [andrew]'s name.
http://www.hackaday.com/2007/07/19/external-laptop-battery-pack/