Ammo containers aren't super common, but you could probably find one at your local surplus store, or on eBay -- and Instructables user r570sv turned this one into a massive Bluetooth speaker with a little DIY spirit and the right parts. Best of all, it's an easy electronics project you can follow too. The full parts and tool list are in the Instructable (linked below). You'll need the right tools, but nothing more complicated than a jig saw and a hole saw attachment for a cordless drill. Once you get everything -- your speakers, your tools and, of course, the Bluetooth module for the whole affair, it's all about cutting the holes in the side of the ammo can, mounting the amp and the speakers and wiring the whole thing together.
Of course, this is an oversimplification -- r570sv ran into more than a few issues in the process (and they're all documented so you can learn from them and avoid them) like picking the right amp -- or maybe a different one than he used -- so you can easily mount it inside the case. Mount and wire all of the components, mount your power switch and power jack on the outside so they can be easily reached, and you're all finished. For the full step-by-step, visit the walkthrough below, and make sure to check the comments too, since a few people offer up some alternative components that can improve the sound in the finished product, and make it easier to put together.
Ammo Can Bluetooth Boombox [Instructables]