I think there are many possible solutions, here I show you a possible way. You could derive the geometry in a new part to do it.The most important condition in sheet metal to obtain the flatpattern would be to have a uniform thickness, the second could be to maintain some regularity in the edges of the geometry (this to apply certain features that otherwise it would not be possible to add them). Notice that I have combined the solids to maintain a single solid, and in this way to be able to make the flat pattern.
Hope this helps, or can give you an idea on how to tackle your problem. Greetings
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Sergio Daniel Suarez
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