An alternative is the car DC to AC converters they sell at Radio shack and other places. Some look VERY small, not brick like at all; you plug them directly into the cigarrette lighter of your vehicle. But I suppose you run the risk of possible damage and warranty voiding to your laptop since it is not approved by Apple. I have used them during a hurricane power outage with both my laptops and suffered no damage however.
I actually have a car Powerbook charger specifically designed for my Titanium Powerbook but I have no use for it. Besides, I'm not sure I trust it with my 17" Powerbook, though the specs are similar. So there are third party makers out there I think. Have you googled?