These are commercially availible, but at a cost. DIY will make it easy and cheaper. Also it's more fun to make your own (unique) model.
Step 1: Materials
- USB-boost module(see below)
- Battery holder
- Charged battery (voltage depends on USB-booster)
The case is to your choice, should be clear after reading this 'ible.(Note: tins are conductive, so prevent shorts by insulation with paper or tape)
Just make sure the battery and boostcircuit fit nicely in or on the case.
The USB boost module the item you'll probably wont have near you, so you'll have to order it.
I bought mine at our University electronics store for 2 euro's.
Note to IPhone users:
An USB-connection has 4 wires: V+(5.1 volts), V- (earth, 0 volts) and 2 other wires. Iphones are the only phones who check the other 2 wires for a certain voltage to know how much current they can drain ("how fast they will charge"). It might be possible that these circuits don't add that voltage to those wires, thus preventing the IPhone from charging.