Tom, since you're looking at Minty Boost may I assume that you need a female USB connector for your new Mp3 player?

If so consider this. I have one charging my Ipod right now. 3.4 ounces with four AAA NiMH cells. It can also serve as a USB powered battery charger and it has a nifty flashlight to boot!!! All for the princely sum of $9.06 shipped. I would not use lithium cells in it because the output is unregulated and the higher voltage of lithium cells (x4) could fry your device. USB voltage is about 5.0 Mnd this would put out 6.8 volts on lithiums.

Also I bought something like this at Wal-Mart for $10.00. Look in the camera section. It's bigger but 4xAA is lots of power for your Mp3 player.

The trouble is, 2 cells (especially NiMH) just don't produce enough voltage to power a USB port. Hence the need for the electronics, which are not 100% efficient and rob you of a little power. But four cells will do the job without needing an electronic boost.

If the above are too large and you have an Ipod this could work. Same idea as the Minty and already built for you. If you don't have an Ipod it could possibly be modified to work with whatever player you have depending on its power requirements. And modifying this would be simpler/cheaper than building a Minty Boost.

Solar charging while backpacking was discussed in another thread. I think the consensus was that it's simpler/lighter/cheaper to just carry extra cells unless you're guaranteed lots and lots of sun all day and you're actually out in that sun (or your charger is) which is hard to do on the fly.