How do you get the hay in the hay ball? Do you have to put it in by the handful?
To the OP, I have a Savvy Feeder, which is the same basic idea as a porta grazer but a different design. It’s very stable on the ground, easy to load, and the horses readily eat from it. It was great when I was boarding because it was way easier for staff than stuffing a hay net.
Now that my horses are at home, I clip a hay net inside a 110 gallon stock tank and put that in my big stalls. The sides are high enough a horse would have to be really determined to get a foot in there, but I understand your reservation. For me though, it’s easier to use this method when I’m weighing my hay and feeding specific rations.