I'm having difficulty modeling a rather complex surface. I'm designing a cylinder head for an engine and am trying to vary the thickness of the bottom plate from 3mm near the edge to 9mm near the central feature(bolt pattern and combustion chamber), the bottom face needs to remain flat. I've tried a loft and a boundary boss (which seemed likely to work, but resulted in a mess of a shape with multiple intersecting solids errors) and now I'm trying a sweep, but nothing seems to work. I think the problem with trying a sweep is that the profile dimensions have to vary, and it seems like a sweep is meant to have a constant, continuous profile. This would be laughably easy if my sketches were more simple and my central feature were more in the center. I'm not sure what else to try.
I basically just need a surface that intersects both the purple and pink curves (highlighted for illustration).
Any guidance would be greatly appreciated. Thanks!