I have a bunch of foam balls the same size as a tennis ball. I suspend them on a heavy string, probably similar to what you did and tie them from trees so they are just off the ground.
I shoot at them with hex heads or judos and they are great fun. I have never had an arrow come back at me ! They are free hanging so I am not certain that this even could happen.
I think the most fun is no targets. . just walk thru the woods or field and choose plants, leaves, and other stuff to shoot at. Some close, some far, some easy, many just impossible. Shoot a lot.
ChuckC