Look around at motorcycle products, e.g. Aqua Box or Dry Bag.
Those might work, but I'd prefer something smaller.
This is how I mount my mp3 player
Do you carry a 'c.phone' with you? If so, how do you keep that dry? All my music is on my c.phone and I do not wanna get that wet. Maybe in a ziplock bag stuffed in my handle bar bag.
I normally keep my cell phone in my panniers, so it's not really accessible, but stays dry. I would think that a ziplock bag + handlebar bag would work. I personally really hate having the headphone cable get in the way so that wouldn't work as a solution for me.