I haven’t done the “knob mod” yet, but I used a layer of the 7/16″ Frost King weather seat that CLC supplied, and on top of that I used a layer of 5/16″ frost king weather seal. you should be able to find it at your local hardware. I learned that at least in my area, Home Depot doesn’t stock it (???) but lowe’s does.
after a couple of trips driving in the rain, so far no leaks.
ken