If you've ever really looked at the vents...they don't stick out but about an 1/8 to 3/16 inch...if that. There is no way to make them appear like the real vents except to use a plastic rectangle with a decal that looks like the vent. It would look really silly IMHO.But isn’t that what the OP is looking for?
It’s your car, do with it as you will.
I asked a while back...... A fake GNX vent that require no cutting. 3M tape back. Or magnetic.....both. Think you could make some shallow curves that mimic the vent.
I know there is a small benefit to cutting holes in your fender but it really does not duct out much heat from your engine compartment they are pretty much for looks. Fenders are not easy to come by and when the parts dry up and the value on the cars go up. Cutting a fender may not be the best choice. The bump on the hood is just for decoration......certainly not for clearance on the intercooled cars. The spoiler on the GN is just extra weight...they are waaay too small to provide any down force at the speed these cars usually go. Also GNX fender flares are pretty much decoration. Many people including myself have run GNX wheel with the same backspacing and tire size as a GNX and there is no need to cut the fender well or need a flare to cover a tire sticking out past the lip. So a decorative gnx style vent would be my choice. Many many Buicks have fake portals over the years. Not asking for a sticker or a decal of a gnx vent. It would have the same rise off the fender as a real gnx flare just no holes. Wish I knew some autocad and could generate one for you guys to look at. Think in a few years when 3D printing becomes afordable I could make one up.