As a start, I would create a cone at each hole and cut them from your block. Or you might do the same with inverted pyramids.
It doesn't involve surfacing techniques, but it does keep the inclination constant and under control.
I would start by making a sketch on the top surface of where your divisions will be (i.e. lines from corners that define whether it slopes towards Hole A or Hole B. Then loft from each perimeter to the associated drain hole.
Why are you using surfaces instead of solids? Not that it matters, same technique, just wondering on the final use and need for surface vs solid.
basin.SLDPRT.zip 178.3 KB