Not going to make you happy, but I found these easy to make.
What I did was cut me out a construction paper template to get the overall dims correct. I just eyeballed them and marked the paper and cutout the template. Used the lower cowl inlet radius to set the overall radius of the large side of the conical.
Then I used the template to mark the sheet of alum. Cut that out.
Then I took the part to a vice with smooth jaws. I marked the center of the tight end of the conical and kept that point constant in the vice. I just closed up the vice jaws to about 1/8" gap.
I kept bending the part a little at a time while moving the large end of the conical all the while keeping the tight end in the same place on the jaws.
Move slow and a little at a time. Once you get it so far, you can just take your hands and form it the rest of the way.
Do not get too agressive or you will end up with little straight sections.
