Agreed. I have used a tire and wheel in a pinch. I also have various lengths 4x6, 6x6 and 4x4 hunks of lumber I can lay on their sides as stoppers. I've simulated a "dropped" load and am satisfied they're safe enough for me. I also have a piece of 4x4 that I've cut and formed that sits under the jack's lift pad so if the hydraulics fail, it catches it.

My preferred M5 jack stand were made of three 14" long pieces of 4x6 lumber bolted together with a 2x12 base and a 2x4 on top for the pad to sit on. I made four of them and they were just long enough and there was just enough room in my short and narrow garage to get the Beast on four them. Don't ask how I jack the rear of the car up.

The only time I got close to being crushed was when I was jacking my Power Wagon. It just slipped off the jack. Fortunately, the tires were still on and there's a lot of room under them anyway. Gets your attention for sure.