riderman,
are you planning to use NiMH cells? You will not get so much runtime from the alkaline ones.
Some 18650 Li-Ion cells (e.g. from DX) would be even better.

I don't know: it's nice to have room in there for the 500A fuse.

Here is my on-the-road charging arrangement for the N900. You could change the connector to suit N8x0.
The small regulator is http://www.dealextreme.com/details.dx/sku.29208~r.60561251