I don't have a pattern but it would be simple to sew up a small bag to fit the requirement.
For a trip down memory lane, we had small purses like this when I was in the Navy years ago and we called them Itty Bitty Ditty Bags...the guys used ditty bags to carry their shaving supplies in...you know, civilian dop kits...and we ladies had to have something to carry mad money around in so the Navy issued us tiny vinly envelope style bags that fit on our dog tag chain and hid in our blouses - they carried a few bills and coins....hadn't thought about them for a while!