If I were you..."whacking" is a no-go. I would put a block of wood under the 2 ends and PULL it down slightly in the middle with a clamp until it takes some of the arch out so it will seal.
As to a book... A fella on this forum who goes by DALORZOF is writing one. It's a DRAFT right now and he is "tweaking" it to get it ready for publication. He's pretty good at answering any questions you might have, on the forum.
My first guess is that the rubber weatherseal that contacts the windshield frame is hard and doesn't seal well. I would replace it. While it's off, lower your top and see if the frame is curved the same as the windshield frame. If it's curved too much it will not squeeze the rubber and seal in the middle.
I WOULD NOT PUT A CLAMP ON IT. If a clamp is needed, other stuff is not right.
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