Help! Need Help Making a Nesting Ring

Hey there,

I have very limited experience in 3d design, but thought this may be an easy project to start. I had my fiancé's engagement ring 3d designed and printed and I want to do the wedding ring myself. All I need to do is make a 3d model for shapeways that has rounded edges and nests against the other ring, and I have the stl file of it. Basically, I just need to outline one side of that. Could someone guide me on this? I can use any free software for it.

The image of the mesh with the green stone is what I'm trying to outline, and the image of the stacking rings is the idea that I'm looking for. I'm not doing anything as complicated as that, but you can see how they wrap around each other, and that's what I'm trying to make, one ring that wraps the one I already have and the curves nest together.

Thanks!

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