Now Caroline uses the rope with the ball attached to it and the V-box.
first video explains the concept without dog
second video shows the dog putting a ball in the V-slot
The foams that represent the little balls are too light.
As already mentioned earlier, it is difficult for a dog to grab something that is next to his head, because by turning his head in order to grab the object, he brings the object further away from his mouth.
BUT, if he bend, so that the ball lays on the ground (and stays their because the rope is long enough, and the ball is heavy enough) he will be able to grab it.
It's a little bit more difficult, but the possible problems can be solved in a next prototype,
with the right materials.
And maybe with some more training, the problems will be solved as well.