Valvet shower valve removal

Help. The shower valves (both hot and cold) have developed slow drips. The brand is Kohler and is probably at least 20 years old. I unscrewed the retaining screw and pulled the handle off. As best as I can explain it, this is what I found. The valve is threaded into a flange which sits proud of the tile. The trouble is that the valve is set in about 1/4" and does not have a conventional hex shape to loosen. Instead there are 2 flat spots on either side. I have tried making a crude tool to loosen it but have not been successful so far. I got a small section of 1" copper pipe and cut it so I ended up with 2 teeth on it on one end. I then drilled a hole toward the other end so I could put a screwdriver through it to get some leverage. Snapped the screwdriver in half and actually deformed the other end. I know that there must be a tool for fixtures like this, but what is it called? If not, what else can I use? Thanks.

Apologies for the picture quality, but hopefully these will show what I am trying to describe. In the second picture you can kind of see the flat spots.

vALVE

It js a Valvet unit and you remove the chrome ring around it to get to the stem unit.

Thanks so much. I feel like the guy who is pushing as hard as he can on the door handle that says to pull. Lol.

door

Or the guy who couldn't remember the work to tell the group how to succeed. He looked at the back door and saw it said "push", so he remembered where to look at that point in the presentation. When he came to the proper spot he looked back at the door, but it was now open and the word on the other side of the door was "pull".