Quick hack: The $1 C-to-D Battery Adapter

http://www.makezine.com/blog/archive/2007/06/quick_hack_the_1_ctod_ada.html

You want to power your gizmo that runs on D cells, but all that you have handy are C cells. In many (but not all) circumstances, you can solve this problem by using the C cell and making up the battery length difference with a few quarters– typically three or four.