I'm not sure how bright you need for this project. I was checking possible LED options, and if 140 lumens is enough (manufacturer's claim, some have claimed that the number is slightly exaggerated), then you might want to consider mounting a 2D Maglite with a 140 lumen 3W Terralux upgrade module to it. Note: this is NOT the Maglite standard 3W LED. Fill it with two 10000 mAh NiMH cells (found online, NOT the ones you'd find at a grocery store). The Maglite is 10 inches long, so it might be a tad bulky, but then the light should just run for 8 hours. In terms of charging, see if someone has a D cell smart charger handy (I hear these are expensive). If not, you can charge them using a dumb charger+timer, or rig something to hook it to an AA smart charger that's based on voltage drop detection.

There seem to be a bunch of Maglites on sale for <$20 these days. The LED is around $30. IIRC, someone sells mounts that would let you mount the light onto the bike. Alternatively, I remember somebody figured out a way to mount only the lighthead, and power the thing with a water bottle battery or something like that.

Disclaimer: I do not have personal experience with riding with a Maglite. Nor do I have personal experience with the LED mod; I'm going by what I've read and so on. If you go with this, I'd strongly recommend doing a dry run before the actual trip just to make sure there are no surprises.