Hello - I would not try to make this from a single surface - make the sides, the top, a transition in between the two, etc. If you are new to surfacing, take a look at the level 1 and level 2 material here:
This thing might also lend itself pretty well to being built from SubD in the Rhino WIP, but that is a completely different process: https://www.rhino3d.com/download/rhino/wip