Looks straighforward (yeah, right, like everything else). I either give it a go, or spend $1000 to have it made (it has to be 40" long, way longer than most). Since I don't want to spend one more penny on this albatross, I'm going for it, and I'm prepared for months of frustration ahead of time. I'm going to make it with three pieces of ally, since a big sheet won't fit in the car.

However, my intake plenum sticks out. How do you make the thing and accurately figure out where the hole goes? Is it just trial and error using cardboard?

could you not get a big sheet delivered? Making it out of 1 piece may not only look better but it would certainly be easier than making 3 pieces and trying to fit them together.

Id try and get 1 sheet delivered. Much easier.

As you say trial and error with some card for the holes.

Measure from the edges of the inlet to the proposed edge of the new bonnet for the hole. Or make a frame for the bonnet out of wire coat hangers and cardboard?

Will follow you advice and go with one piece. Trying to save $100 is a bit silly at this point isn't it?

And measure up and trial and error with thick paper/cardboard it is!
Thank you!!

Make a cardboard one first then you know exaclty the size you need and also where to cut the hole(s).

I made a card template - you can cut a hole, and if it's too big you can stick on some more card until all the gaps are correct. It matters not one jot that the card template looks scruffy, as long as its outline is correct.

My template is still rolled up in the garage rafters, in case I need to make another bonnet (see earlier thread about my flying bonnet!).