Here are the components together.
Here's the sensor in the pocket. And yes, my damn Puma shoes are pink and glittery. Not my first color choice but they are pretty damn comfortable.
I usually wear this sleeveless but you get the idea. I wear it upside down so I can pull my arm up and see it right side up.
Here's the nitty gritty. It's pretty easy obviously. I put several rows of loop-side velcro so it can be adjusted. You just have to be careful because the velcro doesn't stretch. No use putting non-stretch things on an elastic band, right?
You can see the metal buckle I found isn't quite wide enough for the elastic. I made the 1" strap very tight so there is no chance the iPod will slip out.