Onigiri in a Jar

Do you remember the video Maki posted a few weeks back? Where the mother was able to get the kid dressed, fed, and out of the house with a bento lunch in 5 minutes? Well, one of her "tricks" was to put some rice and furikake in a jar; the kid shook it up and out came a perfect onigiri. Well, I tried it. The first ball came out perfect; I was pleasantly surprised. Except the base of the jar was larger than the top and the ball got stuck. On my second attempt, the rice didn't ball up. The third time, I rinsed the jar with some water, then added the rice. That was a little bit better, but I got a rather small ball with a lot of little rice bits surrounding it; it wasn't a perfect ball like the first one. The onigiri in a jar fascinated me. Has anyone else tried this method? (I'm not asking "how to make/shape onigiri." I'm getting pretty good at it.) Any suggestions, other than using a straight edged jar, to offer? Should the rice have been hotter?