Don't bother with the silicone.
If you are dead set on cooking at a lower temp, the first thing I would do is make sure the charcoal bowl and center section aren't out of round. If one or both of them are, it could be the source of an air leak making temp control difficult.
Catch your temps on the way up. When you get to 180-190, start closing the bottom vents down. Always leave the top one wide open.
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