Creating one surface

Hi All,

I am currently designing a gallery in Manila and part of the task is to design this continuous surface. ( not polysurface because I want to use grasshopper for panelling …)
I have used variable surface blend to get the geometry I want ( that was successful) but they are seperate surfaces. is there a way to for me to create one surface that closely follows the polysurface that I currently have? The arched opening in the bottom surface is not necessarily required. I can do without it.

Either grasshopper or rhino solution I can work with. I just need some guidance. Best

surface.3dm (5.3 MB)

Hello - I would say this would be hard to do with any precision, but conceivably possible in a general way by point editing the top surface.