Evil Mad Scientist Laboratories show you how to turn a C battery into a D in a jam… –
Here’s a common problem: 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. There is a 12 mm length difference between a C cell and a D cell, and quarters are about 2 mm thick, so if your gizmo has a really weak spring it could take up to six quarters to do the job.
Yes, commercial battery size adapters are available. They typically cost between 1.5 and three dollars and can only be used for the one purpose– adapting battery sizes. (Well, that, and as a set of matryoshki.) Using quarters can also potentially end up costing as much as $1.50, but it can go straight back into your wallet when you’re finished! Much more importantly, when you really need it, you can probably find a few quarters no further away than your pocket.
Quick hack: The $1 C-to-D adapter – Link.