And ideally to get the best kind of surface quality it tends to be better to build a larger extended quad-like sheet with distinct corners in it and then have extended side walls that then intersect with each other to form the final surface boundary, rather than trying to directly construct surfaces to an irregular outline.

Something more like this:

Try to build the top surface as a large extended sheet, here using sweep but you can use network or loft if you prefer. The key thing is that you construct a broad wide surface that has the overall shape characteristics that you want and do not yet focus on the final border yet because that will come from trimming pieces of this large extended shape away:



Then doing sweep gives this:




Now to cut pieces of that away, your side walls should also be extended upwards a ways as well, here's an example using Loft to build a base block that pushes through the top surface:






Now you can use Trim or the booleans to cut these pieces with each other and trim off the excess area. If you can form a base solid then you can use booleans which tend to be more convenient. Here I've selected the base block and use Construct > Boolean > Difference with the sweep surface as the cutting object. This will slice the block into 2 pieces, select the piece you don't want which is this one:




And then that to leave the remaining solid which has both the side walls and the top extended surface cut to one another:




Using booleans and trimming should be a primary method of your construction when working with NURBS modeling. It will be very difficult to work on things in a "patch by patch" manner like you are currently doing where you're trying to draw in all the final edges of the model to start with and then surface everything directly only to those - especially when you're trying to make rounded outline corners in the middle of a curved shape it tends to be better for the rounded outline to come from a trim curve that trims away some broad surface. When you try to force a surface construction to an irregular boundary it is difficult to get good surface quality.

If your project is difficult to construct in this way using extended surfaces and cutting operations then it can be a sign that the project is possibly better done in a sub-d modeling program rather than in a NURBS modeling program, usually NURBS modeling will incorporate extended pieces with trimming and booleans when it is the easiest tool for the job.

- Michael