Just trace around your cd player and add a seam allowance.Decide if you want it to zip or velcro close,add that,and add a strap.I think a strap like those used on backpacks would be the best.That way it will be over both shoulders and on your back and therefore out of your way and won't be slipping off like a purse does.Does this all make sense???
Kathi

Thanks, Kathi.
The trick is, I need to allow for the cord attaching the headphones to the player to make it to my ears. And all cords need to be away from my arms, or else I will hit them, clutz that I am, while treadmilling.

I see 20-somethings doing this as if it were second nature, alas, I am a 50-something!

Now I have no idea if this would work or not but you really got me to thinking about this project. Here is a link to a jean purse with a flap.

http://www.quiltpox.com/patterns/jean_purse.html

Instead of a shoulder strap sew/glue 2 belt loops (you could use webbing for belt loops) to the back of the pocket. The cord for the head phones would have a place to come out on the side of the flap.

Hope this helps. :-)