Create surface

Heyy could somebody help me? I have a voronoi base and I choose some of them that were the closest to a curve I made, but I want to make a single surface out of everyone, I tried using region union but it doesnt seem to work:( Im trying to achieve something like the blue line I marked, so I could have a base|attachment (10.3 KB)

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Try to “Flatten” the “C” input of Region Union