For electronics projects simple and small, consider using an old 9V battery case It provides a built in snap, easily connecting it to a new battery for power.
When using a battery as a case, keep in mind that: - the inside of the battery should be insulated (with blue masking take in my case) - it is a good idea keep one or two of the AAAA batteries (that can be found inside the 9V battery) and use them to keep the top and bottom caps in place (make sure they are insulated two).
Related: What’s inside a 9 Volt Battery? More Batteries! – Link