Hi Alexander, also one thing that would help for shapes like this is to build the top piece as an extruded piece which then gets trimmed, rather than trying to build a surface with rounded corners in it right from the start (like by sweeping it along side rails for example). So something that looks more like this:




For NURBS surfaces it tends to be better to have rounded corners done as a trimmed away portion rather than part of the surface itself, because having a single surface patch that itself has rounded corners kind of degrades the surface normal in that area.

It's also pretty easy to end up with a self-intersecting surface in the way you have currently done it, there is a kind of "tension" at the end of the surface, see this previous post for some illustrations on how this can easily end up self-intersecting:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1127.4

Self-intersections or near self-intersections tend to be bad for filleting and shelling, it causes the surface normal to kind of spaz out in that area and flip around a bunch. Both filleting and shelling involve creating an offset surface a constant distance away from the original one, going along the surface normal so if the surface normal is going a bit crazy it can cause problems with these operations.

- Michael