Fabricating hood scoop and bad at math... any help?

You could just cut the hole, mock up the air cleaner, put spacers on top of it, then put your scoop "top" on.

Once that's attached, you can just "fit the sides and back to your liking/requirements, take some measurements, and be done. might work easier than trying to use straight edges, since the hood has a curve/slope to it that changes from front to be. it's not flat, so you can't really use a flat edge anyway.