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I can get it higher. Try this:

"Your neighbour or that person over there at the next table could be looking at your screen right now. See that little padlock icon at the bottom that is red? That means you're broadcasting what you're doing right now."

Make it personal. It might not be completely accurate but mostly people are using Wi-Fi these days so it's good enough. But the real problem that leads to people ignoring the warnings is because they simply don't know what they can actually do about it. A warning saying "bad things might be happening" is just clutter if it doesn't tell you how to fix it. So person wants to visit site X. They get a warning. What next? Don't go to site X or make an uninformed choice about whether the risk is worthwhile and carry on. They don't know what the risks actually are, warnings are routine and people mostly think it wont happen to them, so they go to the site anyway.

There are only two ways to fix this. Either make your browser refuse to use a site where the certificates mismatch, no "ignore this" button. Or get things to the point where it is so rare that people actually are spooked by such a warning.

I don't think the second is happening any time soon, though the first would be a massive impetus to bring about the second. I actually would be in favour of the first if public certificates weren't such a money-making racket.