To build this pack, you will need:
-A brain, some logic, and common sense.
- A "size D" four-battery case.
-A pack of Diodes.
- A USB cable to your device.
- A multimeter.
- Extra wiring (just in case).
- Electrical tape.
-And of course, size D batteries.
All of these things (besides the first) can be found at a hobby store (such as Radioshack).
If you feel a little uncomfortable jumping into this, check out this
Instructable on some basics.