I built the battery "box" from the Sony Insiders site tonight. If you have a soldering iron, you can build this easily. Just watch a few soldering technique videos on youtube to make sure you get a good mechanical/electrical connection or you'll be building this again.
Here's the parts I used:
2x RES 6.8K OHM 1/4W 1% AXIAL
2x CAP ALUM 1.5UF 20% 50V RADIAL
1x Battery Snap
1x 3.5mm Female Audio Jack - Stereo
1x Stripped 3.5mm Male Stereo Plug (from old cable)
1x Breadboard
However, I'm still getting 10.8v at the inputs, so it seems like the resistors are not lowering the voltage from the battery at all. Perhaps I should have used a higher value resistor. Sennheiser says the MKE2s (gopro hack) need ~7.5v, so if anyone knows if seeing the full 10.8v from the Powerex battery will cause problems, I'm all ears. I have some regular 9v rechargables in case i need to use those.
Anyway, I'll be attempting to mount it in an Altoids tin with some hot glue. I'll update with pics if anyone wants to see my admittedly amateur work.