I bought a "barebone" MP3 player from eBay (search for "MP3 WMA hardware decoder") that has an SD card slot and an USB connector for USB memory sticks. It also came with a remote control.
Almost two months ago I built a small temporary (until I get a better driver and figure out all the details for the enclosure) blaster from an old studio monitor that has been lying around for few years.
The miniblaster is powered with a TA2024 2x15W amplifier (http://allthemods.com/userinfo.php?userid=10&id=6727) and 10x AA cells.
The self made PCB has a small preamplifier that amplifies the signal for two purposes: for the power amplifier and the other signal line goes to a microcontroller that I'll use in the final blaster for some visualization etc. More info about it here:
The electronics and batteries are going back inside the case, this is just a test setup as I was testing out the MP3 player.