Hi Nick, well I'm not really sure what kind of result you're looking for... is it something like the attached model?

That was made by copying over edges and flattening them, then lofting between them, with the hole filled in by drawing in some lines and using Construct> Planar:



If you then take a solid like this into OnShape or ViaCAD you would have a better chance at shelling it using the large planar face as the opening, if you wanted a thin-wall object you'd probably need to build something like the attached version first in order to produce the final shell.

- Michael