The Neuros HD DC input is rated between 9-16V with .8A draw. Tip is positive. All these parts are found at Ratshack:
RC Pack 9.6V 1600mAh Battery Pack
RC Repair kit (red and black wire connector)
Adaptaplug Model "H" (3.4mm OD, 1.3mm ID)
Adaptaplug 6' extension cord
RC 9.6V NiHM/NiCD charger
 connect the Adaptaplug Model "H" to the Adaptaplug Socket, making sure that tip gos to positive.
 crimp the bare wires of the Adaptaplug Socket into the Red Connectors of the RC Repair Kit adapter. The wire with writing on the Adaptaplug Socket is Positive, and the red wire from the Repair Kit adapter is also positive. Crimp accordingly
 Connect the Repair Kit's plastic end into the fully charged RC Pack 9.6V 1600mAh Battery Pack's plastic end. It only fits one way, so don't force it..
 Plug the free end into the Neuros. On the Neuros you can observe the battery being charged as well as the voltage increasing (press the down button and toggle the "hold" switch)
 Disconnect the Adaptaplug Model "H" from the Adaptaplug Socket. Insert in the Adaptaplu Extension Cord making sure the + & tip line up on the Adaptaplug Model "H" side, and that the tip and arrow match up on the Adaptaplug Socket side. The cord with the white strip on the Extension Cord is positive.
 Recharge RC Pack by disconnecting and plugging into recharder.
It's not gonna blow up if you switch + & -. The neuros has a protective diode inside.
All the parts cost between $45 and $50, including RC pack and charger.
Theoretically this should give the Neuros 2 extra hours of battery life, but I am assuming a bit less. I'll run some tests and give exact numbers.
Here we have the Neuros up top, the RC pack to the left, the Adaptaplug Model "H" middle-left, the Socket (white) crimped onto the red connectors of the Repair Kit adapter in the middle, and finally the extension cord on the far right.
EDIT: btw, this lasted 5-1/2 hours recording a continuous 16/44.1 wav