I have tossed together 6 rechargeable AA rated at 2500mAh, I got 7.2 volts and My Chumby seems to be happy with it. I'm charging them tonight and tomorrow I will see how long they can last at med/high drain.
I figure cycling widgets while playing internet radio will suffice as med/high drain?

So far its dimensions are smaller than the Chumby and the Chumby sits on it quite well, now I just need the diaper

EDIT:2 hours strong so far, shooting for at least three, although two isn't bad at all
I'll update when it finally gives out with a flickering screen or buzzing death noise.
So far the batteries are cold, so theres not to much drain.

EDIT: A little over 4 hours is all I got outta these batteries. Its definitely over three so I'm happy.

EDIT: It pretty tossed together at the moment but I'm gonna house it properly asap.
Check it: http://i28.tinypic.com/5o8a5g.jpg