Yes, I do want that to be smooth, and spliced in a wedge from the tangent points at the top. I found the tangent points by duplicating the back edges and creating a line tangent to them. I don’t know how to do something similar for the surface, so I picked a point on the centerline, interpolated curves on the surface from that point to the tangent points, and used those curves as the edges of the wedge. It is fine for what I’m doing, but there’s probably a more elegant way of creating a surface that is tangent to two other surfaces.
I’ve finally been able to get a closed polysurface from these surfaces, by basically following the procedure above. In the lower corner, I deleted the small surfaces from the original model, untrimmed the surface from the side, and extended isocurves from it. Then I moved the last control points of the extended curves to make them curl in through the walls of the surface below. I connected the ends to make a new bottom edge and used NetworkSrf to replace the original surface. That gave me a good intersection. I deleted the back face and created a new plane that I trimmed from the polysurface created by joining everything else. That gave me a matching back face.