Check out the tutorial over at Instructables. The top picture I actually grabbed from a company called thodio‘s website, where they make the ammo can speakers and sell them. If you have more money than time and know-how, you can pick one up from them for $307 – $430 depending on the specs you want.
If you need an empty ammo can you can pick one up at your local surplus store for cheap, or on Amazon the price isn’t that bad either.
I don’t know how good of sound these things throw, but I imagine it’s probably good enough for the beach or a night of parking lot drinking. Just be prepared to have more guys than girls impressed with your DIY expertise.
Hat tip: Eric, Scott