At first glance the battery compartment looks like any other battery compartment, made up for a
special high priced C cell battery pack. One difference, since it took a "special" battery, there was
a "special" contact on the battery pack. Looking at the rear of the battery compartment, you see
four spring type battery contacts, one on each corner. Under the lower left spring contact is a flat
contact. I took some AA 2400 mah cells, wrapped them in cardboard and put them in the battery
well. In order to make it work, I had to add a jumper wire from the flat contact on the bottom to
the spring contact immediately above.
The only problem I've ever had, since I'm using cardboard rolled up, is intermittent battery contacts.
If I had the time, I'd machine some plastic spacer tubes to adapt to the c cell size. I think you can
buy them somewhere.
Let me know if this helps, it worked for me. Do you need a photo?
Jim