This device is based around the PAM8403 Class-D amplifier. It's a stereo amplifier with 3W of output power per channel. The device is powered by a 2200mAh lithium polymer cell. The battery is charged with a USB cable. An AtTiny88 AVR manages the system and protects the lithium battery from damage. The case will be 3D printed with ABS plastic, and then smoothed using an acetone vapor bath.
I've bought enough parts to make 10 units of this design, so I can sell these to anyone who is interested.