I’ve got a pair of aluminium Robri trims that were a present from my parents as they were a feature of some of the 2CVs I grew up with.
They come with 4mm mounting holes with moulded countersink and I’ve picked up some stainless steel bolts with countersunk alan heads and nylock nuts.
The moulded countersinks are fairly rough and the bolts heads stand proud as they don’t fully drop into the trims.
Using a countersink bit I drilled out the holes so the bolts will sit flush. As the cast aluminium is relatively soft it’s easy to remove more material than required so I took my time and kept checking with a bolt to ensure I finished when it was flush with the surface of the trim.