+1 for the Minty Boost.
I first tried the extender from Brad Culberson site. It uses a 9v battery with a voltage regulator (<$2 at Radio Shack) and fits in a standard Tube Repair Kit plastic box. You also need a short USB cable. It takes 10 minutes to assemble and works OK. It is supposed to provide you with an extra 3 hours of Edge 305 use. Problem is that 9v batteries are not very efficient and the battery get drained just by sitting inside the box.
On the other hand the Minty Boost ($24.55 shipped + a box of Altoids Gums) uses 2 AA batteries so rechargeables may be used. It has a circuitry that boosts the voltage to 5v. As it has a usb port it works with iPod, some cell phones, as well as with the Edge 305. The system should provide an extra full charge from a set of AAs.
I received my kit yesterday. The project is a bit more complex than the previous one but not difficult as long as you have a soldering iron, stable hands, and good vision (tiny pieces). It took me less than an hour to complete it. I tested it on my iPod and my Edge 305. Works as advertised.
Now I need to figure out a way to attach/tape it under my stem and it will be ready for Sunday's Grizzly Peak Century.