If you've only got C batteries for your D-battery device, you can sometimes make up the difference with a few quarters. It's not a great permanent solution, but it will do in a pinch.
For each battery to be adapted, you'll usually only need 3 or 4 quarters (or less than $1!). But since the difference in length between C and D batteries is 12mm (and the height of a quarter is 2mm), you may need as many as 6, depending on the sprung left in your spring.
You can buy battery adapters for around the same price, but they don't go as easily back into your wallet when you're done. Plus, you can probably gather up $2 worth of quarters in a pinch for that emergency kit flashlight.