If you can swing a little more coin, check out the Trailtech stuff. You can get a single integrated HID helmet light for around $130. Now you need 12 - 16V @ 35W for 2.5 hrs, thats about a 7.5A/hr pack minimum. Maybe buy the light and try to borrow and rig up some batteries. I'm thinking four R/C model packs of 7.2V 4A/hr, in a series-paralell configuration should do it, and fit in a fanny pack or cammelback. At a constant power of 35W, 14 V should run at 2.5A give you a little extra capacity, theoretically 3.2 hrs.
Why buy bicycle stuff that you will probably be unhappy and replace it anyway? I suspect you go just a little faster than mountain biker.
