Anyone used coil covers such as the ones from Whiteline (Part number K60951 at https://www.whiteline.com.au/store/)?
Do they work?

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It's coil insulator, not 'coil overs'.
I'm taking a wild stab in the dark, but I'm guessing they would be for hardcore rally cars in an attempt to stabilize temperatures in the springs.
But that's pretty wild, and I can't remember ever seeing them in any articles.

There coil coveers (not 'coil overs' as yyou said - which are shockies with coil springs on em type thingo), you cover the top and bottom loop of your coil springs with them to reduce noise and that.