If I'd made the cut at two angles, the shape would be difficult to transcribe onto the plenum for cutting, so I used a plane controled by three points to cut the CAD model, each point is a measured distnce along one of the folds, so marking out is easy, draw on the points, roughly align them & join with a pencil line.
This is how it looks on the engine. It's not precise, but there's "wriggle room" by adjusting the length of the legs in the silicone hoses connecting the inlet runners to the plenum. A by-product of fitting the TB here is a nice smooth throttle cable run.
So here's what I mean - there's the cut line marked on the plenum & it's obviously not striaght.
But seen from the other angle, it's a stright cut line! It's like magic isn't it?
When I'm feeling brave (probably after the silicone hoses have arrived & been test fitted) I'll slice down that line, cut some 1/4" plate to fit, tap four holes to match the throttle body & weld it together - OK, I'll get someone else to weld it together.
But if it works that's another major hurdle crossed.