Drawing Curve Inside Shape

I am doing a 2D drawing in Rhino. How do I trace a curve inside of a flat shape? I need to create a closed boundary (for hatching). Is there a way I can click inside and get a full trace of the inner perimeter?

My first guess would be CurveBoolean, but without a specific example it’s a blind shot in the dark.
Can you post an example?

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If you flat shape is a surface or polysurface try DupBorder.