Bruce Mueller was looking for a way to charge his phone for a couple of weeks while off the grid. So he combined the Mintyboost from Adafruit (designed for two AA batteries and usually tucked into a small tin case) and combined it with his D-battery Maglite, for higher capacity charging.
With a little tinkering, you could create your own. Bruce writes:
Removing the bulb and reflector, I created a space to hold the mintyboost board. By disassembling (breaking) a bulb, power can be drawn from the bulb socket.
I sliced it in Makerware, medium quality (.27 layers, 2 shells, 0 per cent infill). It sticks to the plate like glue — take your time and a nice spudger to get it off...
He also offers the 3D plans for the holding bracket.