I've been investigating this for building some, but can't beat the price of:
http://www.powerstream.com/dc6.htm
connected to a DR-400. Just run it at 7.5V, since the BP-511 is a 2 cell Li-ion battery with a nominal output of 7.4V. The 8.4V output of some units probably just provides a slightly more efficient power conversion in the camera.
Eric