I have not gotten anywhere near ready to put a belly pan on my car, but I do have one product that may help. For another project, I found some self-tapping metal lathe screws at the hardware store. They've got a nice drill-style head and did fine piloting their own holes in 1/8" thick angle iron. The heads are wide and low profile, good for holding coroplast without an additional washer. They are apparently for holding fine metal lathe against the side of a house before applying stucco.
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