This is my shot for the new design. the working is simple. when the knob is rotated, the rubber plate slides over the lift plate n is gradually lifted on top, thus opening the vent gradually. the pointers above n below show how open or closed the vent is. the inner gallery for the top cover takes care of random splashes
STL (Stereolithography, filesize: 101 KB), IGE (IGES, filesize: 2.92 MB), SLDPRT (SolidWorks, filesize: 1.95 MB)