This is a wrench/knob that makes removing a carbonation cap easier. A carbonation cap is used to make soda water at home. It fits onto a standard 2 liter soda bottle and adapts to a CO2 tank to allow you to carbonate your beverage.

These caps are smooth stainless steel so once it gets some condensation on it, it gets slippery. This wrench fits down into the grooves on its surface to allow you to wrench it off with a handle. You can just leave the wrench on the cap permanently as though it were a knob. You can carbonate with it on, it will not get in the way. I printed mine from ASA but I'm sure any rigid material will work.

I'm not certain if this design is universal, but judging from pictures off amazon I think it might be. The specific cap I am using is this:

https://www.amazon.com/Stainless-Carbonation-Pressure-Barb%EF%BC%8CCO2-Carbonate/dp/B01K4GGYT0/ref=sr_1_6_pp?sr=8-6
 

I made this using Onshape. Feel free to redesign the model for your own purposes.

https://cad.onshape.com/documents/f159a3b100fc6f0d37dfc6e4/w/1800fdec11c54d02f57fbf5d/e/d41d635eab703c2ae632bb10?configuration=diameter%3D0.032200000000000006%2Bmeter%3Bheight%3D0.019%2Bmeter%3Bknob_distance%3D0.02%2Bmeter%3Bknob_number%3D2.0%3Bthickness%3D0.004%2Bmeter%3Bwing_diameter%3D0.0265%2Bmeter%3Bwing_height%3D0.0034%2Bmeter&renderMode=0&uiState=69179c7076021baf6eef5cad