How to get the sandbox to do a 'complicated' face?

I’m trying to get a sandbox to create a mesh for this space, so that the top edges of the faces are the ‘border’ or ‘ridge’ of the slope, and the lower section is the base. The issue is that because there are 2 90 degree turns, it’s not lining up right. How can I overcome that? Here is the model, and here is what keeps happening because of the angles.

Will you share the file?

I recommend you to use a different tool, curviloft or soap skin bubble could make a better mesh.