Maciek -

Filling the gap in the band -

Making extensions and capping off after the fact can be difficult, if not impossible, on complex objects. It's best to avoid making patch work by making the object larger than needed, then cutting away what is not needed.

I would delete the Loft and add one more profile curve (planar) inside the watch. That way you are making the band longer than needed. Try making this new profile curve a copy of the curve that conforms to the watch opening, flatten it using the selection handles (to make it planar), and stretch it a little taller, and wider than the watch. Adjust this curve so the areas that intersect the watch conform to the watch opening.

The resulting Loft should be be a solid because each end is planar.

Now you can create a cutting curve in whatever shape is needed to Boolean away the unwanted band section just inside the watch. The result after the Boolean should remain a solid.

It may be that the band does not conform exactly to the watch opening. If so, you could take a version of the watch that has no opening, and use the watch band to create the opening via Boolean Diff.

How accurate you need to be depends on your application. It's really difficult to try to replicate complex semi-organic objects to exactly match a blueprint.

Ed Ferguson