I would use Bumpersoft.

Thanks Cactus. I'll go with filling in those diamonds as I don't want to cut my original. Do I use modelling clay from art stores for that?

The aluminium honeycomb grill used against the carbon looks mental.

You could cut it out, but it has to be a very neat cut.
You could also fill those diamonds and cut the hole out of the part. Then quickly rub the edge with some 240 grit to give it a slight radius. A coat of clear will naturally round it off a bit more.

Hello,
I'll get straight to the point. I'm trying to make this OEM grill with faux vents into this CF one.
When I make the mould, should I cut the original so that when I lay the cf it has an inner edge to bevel around?

OEM grill


CF grill


Where it'll go