To make the key end I attached a 10 inch abrasive cutting wheel to an arbor and manually worked the end of a 1 inch piece of pipe against the edge of the wheel until I had a slot that would fit over the shut off valve in our water box. That would have been sufficient, but I wanted to add a little leverage and make the key easier to align on the shut off valve. So, I welded two small pieces of strap iron onto the end of the pipe.
I have not needed this water meter key often, but it works very well. And, I used up some scraps I had without a penny out of my pocket (other than for welding electrodes).