Yes, there are portable ammo box speakers on the market, but why bother spending all that money when you can make it yourself? DIY website Hack n Mod will teach you how to make one for under $50. With a .50 caliber ammo box, a set of speakers for portable devices and a couple tools, you'll be set on creating a portable ammo box speaker in no time. While it may be a cheaper alternative, be warned that this DIY speaker does require hours of hard labor.
For instructions on how to build the portable ammo box speaker, go to the Hack n Mod website to download the instructions.