After humming and hawing over a bunch of online netting, pads, and gizmos to stick in my garage, I ended up doing about the most low cost thing you can think of, and it works great. Two eye hooks in the ceiling, attached a rope between the two, and draped a heavy duty old moving blanket I had around over the rope. Completely deadens the sound, no vibration, when using baseballs. Good for plyo balls, but for that I concocted a soft foam/ 3/4" plywood/soft foam/ 1 1/2" puzzlemat sandwich that I attached to the wall just behind the moving blanket. Plyoballs return better, and 57 jr, can practice ground balls with a baseball. Not the best, but keeps his hands in shape.
Because of the long winters up in MN, we are at a huge disadvantage at the best of times for getting in baseball training. With the gyms closed it's even worse.
Next thing up is to build a pitching mound.