You want the openings in front to be about as large as what you want to get cooled. They should be in front of what you want cooled.

Best case, you want the outlet openings to be about 1.5 times larger than the inlet openings. And they should be behind what you want cooled. If they are in an area of negative pressure, you're lucky. If not, they need to be larger than 1.5.

My Hellcat has outlet openings that're probably just a touch larger than the inlet. My Corsair has maybe less. The Corsair flew through the last 2 summers without a whimper. The Hellcat only saw a month of heat. It was new the end of this summer. But with it's cowl flaps recently opened, I'm betting it'll have no problems next summer. The Corsair exit openings are in areas of negative pressure. The Hellcat has openings that had negative pressure created for them. The picture shows one of the "scale" cowl flaps after creation. They worked well on the fullscale, and will probably work good on mine.