Now lets get to that other planter. Isn't this the weirdest cat litter box you've ever seen? The concept is that any litter the cat kicks up the sides just slides back down to the bottom with the rest. The reality is that cat just kicks the litter out of the front and back of the littler box instead. *sigh* Oh...and my fat cat tries to get out the sides and tips the whole thing over. Epic fail! That's ok, all I had to do was drill some drainage holes in the bottom, add some soil and the mint I got from our seedling exchange at work and I'm good to go. You could probably use just about any container you have laying around for mint. Just don't plant mint directly in the garden without some way of keeping it contained, or you won't have anything but mint in your garden after a season or two!
Oh, and when you're choosing your planter make sure it's not cracked. I wouldn't use ceramic because it can crack if it's left submerged in the garden over winter (learned that the hard way!) The runners can get out through the cracks and you'll suddenly find new mint plants popping up all over the garden.
I should mention that with a planter as shallow as the cat box, the roots will freeze in a zone like mine. (5B) When the roots freeze tight like that, the mint will not come back the next year. To prevent this killing freeze, you can transplant the mint into the garden a few weeks before the first frost is expected. Mulch heavily. Transplant out again right after the last frost of spring. It's a pain in the butt for sure, but it wont take over your garden this way!