I can't tell you the exact size I used, but I just took a piece of the trim in question to Ace Hardware with me and found the perfect little flat head stainless machine screws to slide right in there.
I wasn't sure about it spinning when I lightly tightened them down, but they didn't since the screws/threads were new.
You can also get screws just a bit larger and grind a flat on each side of the heads so they don't turn when tightening.
Don't forget to put sealer on them or they can leak water into the trunk.:D
Like this but smaller.
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