My daughter's lion flashlight runs on two C batteries, but all I had were some AAs.  I knew one can buy plastic shells that AAs fit into so you can use them in devices that use C batteries.

But why buy plastic adapters when you can make them out of scrap cardboard and a small piece of tape?  

So, I took a corrugated cardboard box that was going to go to recycling anyway and cut a strip of cardboard.  The width of the strip was equal to the length of an AA or C battery (it's lucky that they have the same length--or close enough), not counting the terminal that sticks out.  I cut the strip at right angles to the corrugations, and chose the length so that when I wrapped a AA battery in the strip, it had approximately the same thickness as a C battery.  (The length you need will depend on how thick your cardboard is.)  I then took a piece of tape to tape the strip in place, and now I had some ugly-looking "C" batteries which I put into my daughter's flashlight, and they worked great.

I wouldn't do this on a device that draws really high current, or on one where one would worry about the cardboard catching on fire.

These particular AAs weren't rechargeable, but this would work great for using rechargeable batteries in a C battery device, if your household is like ours and you own lots of rechargeable AAs, but no rechargeable Cs.

After posting, I realized that what I did isn't very different from this, but I think corrugated cardboard makes things go much faster, since there is less wrapping around.