I am having trouble converting a solid into sheet-metal. A kind of sweep into a curve that I can then flatten.

I have a part here that is going to be rolled into a curved cone. It is a dumb solid made from two other parts, but I can't seem to make the new part into a sheet metal part that can be flattened. I am attaching the best attempt. The default configuration is how the final piece should be like. The SheetMetal configuration is my best attempt at doing so.

Please help?