Struggling to convert a shell to a solid

Greetings folks,

I am very new at 3D modelling and have run into an issue I’m afraid is way outside of my knowledge to resolve. I am trying to create a 3D model in order to 3D print it. While the render preview in Rhino is exactly what I want the object to look like, I am unable to have this success transfer over when slicing the .STL file with Ultimaker Cura. I believe the issue is that I only have a shell, and not an actual solid. Furthermore, every method I have read about to create a solid in Rhino under these conditions has failed, and I believe it is because the original model I modified to make my model has gaps/unconnected lines, which prevents the formation of a solid.

Is there an easier (or at least less tedious) method of fixing this issue than completely remaking the model and ensuring there are no gaps?

Please let me know if more information is required. Here is the file I am working with:
problemchildbig.stl (10.3 MB)

Hello F D,

Could you attached the .3dm file as well? That way we can see what is going on closer to the source. Thanks!

Certainly! I wasn’t able to upload the .3dm file through the forum due to its size, but here is a Google Drive copy. problemchild.3dm - Google Drive

Thank you so much!

Hello F D,

You have a lot of naked edges (command ShowEdges):

Upon further investigation, it looks like the model is actually a bunch of different pieces, none of which are actually solids:

You would need to join all these shapes, close any holes, etc. to make a solid model that could be printed. Keeping everything clean can be difficult. Does that make sense?

Yes that makes a whole lot of sense! How did you manage to separate the shell in the second picture? I think that understanding how to do that would help me a lot in resolving this issue.

Thank you sincerely for your help!

Certainly. Many of the parts seem to be in (often nested) groups. So I went through and used the ungroup command to separate everything out.