Leaks around windshield gasket

This new windshield gasket came from Precision. Since then I've heard that gaskets from some suppliers fit better than others. The bodyshop had a 'specialist' install it.

The old one fit about the same, with large gaps, it leaked too. Water is coming in at the lower corner and dripping on the carpet from the edge of the dash.

I want to fix the leak myself without removing the windshield again. So far all I've heard is not to use a silicone product. Anyone with how-to advice? Thanks!

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