Thanks Pete! I have struggled to find off the shelf vents that are really big enough and the right shape. I hadn't seen 280 vents before, there seems to be quite a variety of useful and good looking vents.
Because of the hood stiffening frame it's difficult to get large enough vent holes, so my solution is not ideal. I will leave a small open strip at the bottom of the radiator cowl, mainly to allow rain water to drain, still allowing some air to exhaust under the car. My main aim is to minimise the volume of air exhausted under the car.
I considered Ford Focus ST vents, but Mark, my fabricator reckons he can make bespoke louvres. The rear part of the vent, which is actually a rain protector for the strut top, will remain open as I like it as a feature.