Hi Guy, I deleted this current kind of patchwork of multiple surfaces:



So it looked like this:



Then I used Merge on the bottom edges so that there was just one edge down there:



Then Construct > Extrude which will now make just one larger surface going across the whole side-wall area:



Then Edit > Trim the large side-wall surface with the top edges:





Then Edit > Join to glue the pieces together.

- Michael