Newton Blaster attached to a vertical beam near a door. The cover is intended to keep users from grabbing the wire and pulling on it. It works well. My first version of this did not use a cover, and the wire was in shambles after a few days. The original wire used was not piano wire.
Piano wire is very strong.
The recoil block works fine for the distances here, but a block of metal might be useful. Astro Blaster balls were used, but it might be better to use nylon balls, especially for the top ball. The hole in the top ball from the AB has a fairly large hole in it, and that may cause some difficulty in it sliding up the wire. I have no source for nylon balls, but you might try Small Parts, Inc. in Florida. Two by Four Pick a straight two by four, both from the side and front.