[dragula81]

My thought is to just flatten the end in the shape of a slight oval just enough so that it can't come back out on it's own. I'm not sure how much excess pin will be sticking out of the bottom on your car but most I've seen have 3/8-1/2" usually. That should be enough to flatten it with a hammer or if you have access to a press, that would probably make it look better and more uniform if that's important to you.
You probably wouldn't be able to "mushroom" the end of it if you tried anyway. I would think that the pin would probably bend first.

Good luck!!

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