I just knocked this up for myself so my loose change wouldn't lose itself at the bottom of my bumbag, but some friends saw it and wanted one (and insisted on paying a quid each. Hope the Inland Revenue don't notice. :)
It's pretty trivial: a rectangle of nice stretchy spandex 3"x7.5" (80mmx195mm), folded in thirds (both edges to the front) and sewn along the edges. Oh, then you turn it so the seams are inside (and the sparkly side is outside). You probably want to use fabric that stretches half as much again in both directions (50% four-way stretch).
Easy enough to use:
Hold in your hand, with the loose edge at the bottom and facing you.
Using your other hand's thumb and finger, stretch the loose edge up and backwards over your fingers, so the pocket's securely held.
Root about for your change - you can pick and poke in the pocket, or you can tip all the coins out into your other hand, then stretch the lip wide open to slide them back in.
Now use your other hand's thumb and forefinger to pull it back off your fingers, to close it.
The only potential problem: if you turn it upside down twice, all your change might work its way out and fall on the floor. So be careful when you pull it out.