Hey Ben
I know this is a more of summery build, but since you are building something with parts around the shop I thought I would suggest it.
My idea is a modified boombox that has its tape deck or cd player taken out and instead has a 3.5mm cable and usb/ipod plug for power.
There could be a slot for the Mp3 player to slide in securely, but can still be accesed to change the music.without removing it.
You could also replace the space that every boombox has for batteries with a rechargable laptop battery or similar.