You shouldn't use silicone anything on a steel-bodied car. As the silicone cures and ages it releases acidic compounds that will initiate corrosion on any unpainted steel it contacts. The silicone windshield sealants are real bad about this and causing rust in the windshield channel. OEMs and glass shops use a gooey urethane sealant to install glass.
I'd go with the vinyl decal with some high quality 3M pressure sensitive adhesive backing or possibly a plastic plug like from
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