Well, you can still use math to size the opening and determine the actual air flow requirements etc. Keep in mind though how many aircraft are built by hand. Home builders use simple tricks like this all time especially using fiberglass or other composites, especially before CAD was the norm and people had to "loft" wings and compound shapes with fricking pencils. lol.
I am thinking of making a scoop for our 65 Valiant that looks like the 66-67 hemi scoops. Low and wide. I happen to like that look and I believe if the opening is close to the front, it won't just be cool looking (or not) but should actually catch some air.