at the risk of sounding completely ridiculous.....

I do alot of commuting by bicycle and I had an asinine idea the other
day - embedding a small speaker into my old,tattered courier bag to play
music(duh) without impaling myself on a pole from having the headphones
on. It would presumably connect via 1/8th jack to my CD player ,etc.
Ideally it would be unpowered or by batteries. I realize this limits what
kind of volume I can get, but it don't need it horribly loud.

The problem : I have no clue what I'm doing. I'm reasonably experienced
with electronics, but nothing speakerwise. Any suggestions, assuming
this is even possible within a reasonable price range? Is this completely
fatuous or might be possible with a certain product?

thanks
dave