Lift the gray plastic, and take another photo. I don't remember what the gray thing is called. But you need something thinner, unless that flange lifts and pushes down. If it is not stable vertically, it should be secured vertically.
If the flange is stable vertically, and slots are not messed up, use wax.
If the slots are messed up, Sioux Chief 886-MR or
Harvey's 014710 (which is stainless) can provide the good slots. They add a little thickness, but not a lot.