Last summer I decided I wanted to build a portable stereo speaker box so that I would have something to play music with at the beach. I settled on using a surplus metal ammo can as the enclosure, and picked up a 50 cal can at the local surplus store. I wasn't going to repaint it, but the can I purchased had been rather poorly resprayed at some point so I sanded it down and painted it with some tough paint. I bought a 15w Class D amplifier from Sure Electronics. These amplifiers are small, powerful, extremely efficient(~90%) and above all, cheap. I also purchased a set of 4" automotive speakers. I had some 3s LiPo battery packs on hand already, so I installed the appropriate battery connector on the amplifier. These batteries seem to work great with the amplifier since it operates well at 12 volts, and ceases working just before the batteries reach their minimum charge level(~9v), ensuring they don't become overdischarged. They also give a working time of around 8 hours with a 2200 mah pack, and there is certainly room for a larger battery if longer runtimes are desired. There is also room inside to permanently install a small balance charger if a complete system is desired, but I just take out the batteries and charge them outside. I put in an audio input jack, power switch and a power indicator I salvaged from an old control panel, and the final product is an attractive portable amplifier that has served me well for playing music on the go.