UPDATE: I have a newer version that is a bit easier to assemble and slightly less material. http://www.thingiverse.com/thing:1508063
Never be caught without change for a shopping cart again.
This model easily attached to any keychain or keyring and holds only 1 coin.
The attached files are designed for a Canadian $1 coin but it is a customizer so you can adjust it for quarters of other currency.
I found that for my Dremel printer with Dremel PLA using the free Print Studio at 100% fill this print is perfect, no gaps, nice and snug fit. Slightly stiff to put together, cleaned it up a bit with a blade (very tight fit) and put it in a vice and crazy glued the back seam.
UPDATE: Posted an older version without some of the setting to make printing easier. Set "ForPrinting=1" to have the model spaced for printing.