http://i.imgur.com/35fujx3.jpg
Just remove the felt tip and the black ink section of a Sharpie. Keep the cap, and use the cap and clip to store the o-rings. Keep it on the back of the Sharpie like the photo, and the clip will retain them, and all you have to do is slide your senko style worm into the open end, and slide your o-ring on. Don’t spend $10 on a tool you can make yourself.
Now, just have to find a place that sells o-rings cheaper...