Hi Warren - for the first example of a box with open ends (with the screenshot above), that one should have planar ends and you can cap an object's open planar ends by selecting it and running Construct > Planar. That command has 2 modes that it works in - if you select planar curves it will generate a planar surface as output and if you select a surface object and then run it, it will look if the surface has naked edges that are planar and will create planar surfaces there and also join those in to the object to seal them off.

If your opening is not planar then you might try instead building a bottom surface initially instead of trying to build the side walls initially, then after you have created your base surface extrude that surface instead of extruding curves.

Also instead of trying to cap a non-planar opening usually it is an overall better process to first build a more simple blocky shape that uses planar curves and extends a ways past the final area, then use a boolean operation to slice that initial block shape up with a cutting curve. That's kind of a key adjustment to make as compared to poly modeling - with NURBS modeling try to form more of your shape using all 2D curves with some of them being applied as cuts to previously constructed pieces. If you're trying to draw everything all as 3D curves only and then trying to fill in everything between those, that's sort of more of a polygon modeling workflow instead of a 2D curve driven workflow.

So for instance, to do a shape with a non-planar top don't try to work "patch by patch", instead build a straight shape initially and then cut it like this:












If the top is not straight in the "side view" direction and is curved all over you still want to follow this same kind of process but instead of doing the cutting with just one side profile curve you construct a cutting surface using sweep or revolve or whatever, and then use that surface to slice the base object up.

But you should try to be working with a simplified solid base object more often like this, rather than trying to fill in things piece by piece, if you're not building extrusions with caps on them right from the start you're kind of going in the wrong direction at that point already.


Really a key idea is "extensions" - often times you should be initially building an initially more simplified extended base shape. Don't try to build to the final 3D edges right at the start of the process, non-planar edges should come from an intersection result rather than directly drawing them in.


Hope this helps!

- Michael