I installed my vents yesterday. I included the white insert that they came with (purchased from CLC). I noticed that the covers don’t screw tight and I also noticed that the manual doesn’t seem to use these inserts.
see page 373, CLC mentions they elected not to use the insert either. Take that white insert out, and line the inside of the “cap” with some self adhesive foam weather stripping. it should seal tightly against its self. The cap should not rub against the outside of the shell though.
I used the inserts and consider them important. They prevent rainwater intrusion into the teardrop. It is the same concept as a dorade box in boats. The insert prevents water entry from above. Water won’t roll uphill. If you use the CLC method you can’t open your vents in a rainstorm – which is exactly when you want the most ventilation.