We went camping with some friends who had small kids. They brought out the ladder-golf and instead of golf balls they used tennis balls. Thinking this strange I was about ready to ask why not golf balls when I looked over and say his 6 year son old getting ready to toss one of the sets and on his back swing boinked the back of his head with the tennis ball. Question not asked was answered.
I made one out of some PVC and found some used golf balls at Wally World, cheap. I set up a drill press and drilled them. A word of caution when doing this, wear goggles as I found one with a liquid center and got it in my eyes when it blew.