Valve to control water intake from skimmer and bottom. How to lock?

The valve in my skimmer into the filter can be twisted to control the flow between the skimmer(top) or the intake at the bottom of the pool. It basically looks like a little nozzle with a little 1" knob sticking out of the side. The knob lies flush against the bottom of the skimmer. When I pull the valve out it's this hunk of copper with 'AMERIVALVE' on the side if that lends any clue as to what it is.

I set it to where I want just enough flow to have my crawler slowly go around the bottom. However, after a while the movement of the crawler causes the valve to turn and increases the flow into the skimmer and the crawler starts moving way too fast.

I need to figure out a way to lock this valve to where I want it. I currently have a concrete brick in the skimmer so some weight can be put on that little knob to keep the valve from rotating. However, it doesn't work all that well. I bought a plastic device from a pool dealer when I asked about this. It has two legs and you can unscrew one of them so it's wedged solidly on the inside of the skimmer. However, I can't figure out how to secure the nozzle or knob to it.

Any ideas appreciated. Thanks!