I cut out the template for the airvent and placed this on the outside skin, marked my holes where I wanted them in relation to the plastic scoop and predrilled the holes through the skin first. I figured this way would be better as the plastic is very soft and I didn't want any errant run drill holes. It was just a matter of placing the scoop where it needed to be and match drilling the holes. Dimpled the skin, countersunk the plastic carefully, made small spacers out of spare aluminium, sealed between the scoop and skin with the same high temp sikaflex I use on the brake reservoir on the firewall and pop riveted with countersunk rivets. All good.
