Step 1: Parts and Testing.
5v regulator (Lm7805)
Female USB port
Braided wire (20g)
Used 9v battery (Walgreen's brand)
5 minute liquid epoxy
Non-conductive epoxy putty
The circuit design is from http://ipod.hackaday.com/entry/1234000270029372/ All I did was add an LED so I could see if the battery was good. He's got nice diagrams of the USB and regulator to help you wire this together.
Step 2: Disassemble the Battery:
I chose to use a Walgreens brand battery because it has nice thick plastic on the top and bottom. Duracell uses paper.
Step 3: Solder the Circuit Together:
So the USB, LED, and resistor are on top and the regulator is on the bottom. The wire lengths are just to have enough room to epoxy the top in before the bottom, 2"-3".
Step 4: Prepare the 9v Case:
Cut the case at about the 2/3 mark.
Score the inside of the case with a file or an awl to give the epoxy some kind of mechanical hold.
Step 5: Epoxy Putty the Top:
Once the epoxy putty has cured (not fully, just enough that it doesn't deform when you push on it), put the top half into the case and seat it against the lip. While holding it against the lip drip a small amount of liquid epoxy around the edge to hold it in place.
After the top is held in place fill the remainder of the case with putty, leaving just enough room for the bottom to be pushed in. Push the wires into the case and force the top into the putty 1/16"-1/8" past the edge of the case.