[sourdough]

How was it sealed originally as OE? I think you could do it a few different ways; put butyl tape under it, screw it down and trim the butyl; do the previous then seal the edges with some pro flex or just put the vent back on and seal the edges with pro flex. Whatever you do I would not use silicone - it is a pain to work with to install, remove or maintain. You can get the pro flex in various colors to work with whatever you have. If you can't find pro flex you could use dicor non sag as well.