The upper rim of the object is made lofting two copies of a curve.
This is to be booleaned into the main body later.
The next two horizontal curves are; a copy of the curve used in the upper rim, and a blended curve below it that was then pulled to an extrusion of the upper curve so as to make them align vertically. (This generates a large number of control points on arcing section of the curve. I used the remove multi knots tool to reduce them)
With these I used a 2 rail sweep to avoid a creating a polysurface. (Just because)
Next, having two surfaces to work with - The vertical face of the rim and the horizontal surface of the base, I use the blendsrf tool set to the curvature level and add shapes at the quadrants of the corner and another at the midpoint on the longer side.
Afterwards I use the remove multi knots on the surface.
Shown are the results with Zebra.