We practiced again today with the large tennis ball. While the success rate was not huge, and she still had some trouble catching the ball, I feel pretty good about following this path. I didn't use the perpendicular prop today, but even without it she was getting some really nice height on the box. More importantly, she sustained that height throughout the entire training session. She had some trouble getting the box to trigger, particularly when she went really high, but we ended up with just 2 no-triggers, 3 triggers with drop/missed ball, and 3 triggers with catches. I think the fact that the larger ball's position on the box is further out to the side (vs. the small ball hole, which is a couple inches closer to the center) also helped her to maintain height in the turn. And the large ball is just a bigger, brighter target for her, so in the long run I anticipate the success rate will go up.
I made a short comparison video of the best turn from our August 6th session vs. the best turn in our session today, just to make sure I wasn't just making things up in my head. And sure enough, the difference is clear.
Here are side-by-side still shots, too. Given that the small ball turn was never very high, and degenerated further in later training sessions, vs. the large ball turn which is nice and high and so far is just improving in our one subsequent session, I feel like we're on the right track.
And if anyone's interested, here's the full video of our training session today.
It's possible...throwing salt over my shoulder, knocking wood, and making fawning gestures to the doG gods above...that this is the baby box turn I've been searching for, and now I can only hope I start seeing it more and more.