Fitting an Interior to the Exterior

This is a problem has bugged me for a long time in various contexts so…

If I have an interior that is oversized to fit an odd shape

And I have an odd shape exterior

How can I reliably trim off the excess and combined the two part?

All too frequently, when I do do a boolean difference with the two, that works. But when I remove the trimmed parts and try a boolean union that fails.

The problem surfaces tend to have been created with SHELL. That might be a clue to the issue.

Then I have to do all kinds of tweeking and hope to find something. Is there some approved solution to this?

Duplicate the edges of the extents of the smaller shape and use those curves to extrude and trim off excess