Wow, here’s a really cool DIY on how to make a Tennis Ball Massage Chair that will get leave you feeling fully relaxed. Even better, you could probably substitute any type of ball such as golf balls to get a different effect.(softer or harder)
The tennis balls you see are not glued in place; they are held only by different-sized holes in the top and bottom sheets of plywood. The holes on the bottom are smaller so the balls don’t push through when you sit on it. To create the contoured effect, the sizes of the top and bottom holes vary in specific ratios. The balls can still freely rotate and some of them can come all the way out. That said, it is much easier to make without all the contouring, but decidedly less comfortable. I made prototype version in which the balls do not contour, and will include some pictures of that chair at the end of the instructable.