Ace didn't have the size I needed, so I'm having to improvise. First I tried a rubber band (not beefy enough), then I tried one of my daughter's pony tail holder widgets (better, but too loose), and finally put a zip tie on it. Hopefully I can make do with using zip ties until I can get an O-ring.

Side note: I was wanting to mount the battery under the stem, but that did not work out. Then I tried under the top tube, but it would have interfered with my control cables (brake and both shift cables are routed on top of the tube). Luckily, I managed to fit it behind the head tube.