[The Menta] is a great little Arduino clone that fits in a mint tin yet still has Arduino-standard header sockets for shields. I found the mint tin limiting though if you’re adding wires & parts to your circuit, plus I have always felt a little iffy about putting prototypes in metal enclosures. So here’s a printable case for mint tin-sized circuits like the Menta. It has openings for the DC power jack and the FTDI cable.
TodBot has both 25mm and 35mm versions up on Thingiverse but you could easily stretch the case enclose almost any circuit or shield you need.