Hi Brian, if you extrude a non-planar curve MoI won't know how to cap it automatically.

But if you extrude a surface, it will know how to cap it because it can copy the surface to do that.

So in the case you show there, I would recommend using Edit/Trim to cut the surface using those outline curves. Then you will have some small surface fragments - select those surface fragments and extrude those, rather than extruding the curve outlines only. That should produce the capped result like you want.

- Michael