Hi Mark, I've attached here the result of building the extrusion in the other direction, is this what you needed?

There are a few kind of tricky steps for extracting the 2D profile curve, because when you flatten half of a 3D curve you can get stacked up control points at the end regions, so you need to do a few different steps like using the Rebuild command on the curve to simplify its control point structure and make sure that it is exactly symmetrical, and then remove a couple of stacked up control points from the ends after squishing it flat.

Maybe in the future there could be some command to help with this, and in fact I do want to make a command that can fit a surface through a generic 3D curve - but it will be a pretty tricky thing to make that command recognize that 3D curve could be fit with an extruded surface from either direction instead of just squishing a more arbitrary surface to it.

- Michael