Here's what I got off the top of my head...
- 12 V DC > 120 V AC adapter for the car
- Extra batteries, although that'll probably take 5+ medium-sized packs at $15-20 a pop for an older laptop I have lying around. (Lasts maybe 4 hours on a pack of moderate age)
- Some DIY solution jerry-rigging something together with parts from China.
All the fancy solar stuff seem's pretty pricey.
Honestly, I don't think I've come up with anything you haven't thought of already though. What kind of mobility, terrain constraints are we looking at so far? Something you can stick in a backpack, truck bed, next to a secluded cabin?