Add draft from 3D curve?

With SW this is easy with Extrude from 3D sketch with draft. The entire 3D top curve below can simply be extruded with draft to bottom face.  Onshape does not yet support 3D sketches or extrude from 3D curves. I know parting line draft may address this when released. (ex discussion here ) - https://forum.onshape.com/discussion/comment/29874#Comment_29874 

We are in the pattern & mold business and this is needed all the time.
 
Are there any reasonable workarounds in the meantime?  OS support has suggested sweep but the profile 'twists' and the result is not what is desired (see image below).  I have a sample part here - https://cad.onshape.com/documents/e794c9cce4fc26fb6ca3c849/w/8fc4464aec86e2d7ab4356c9/e/2d5a1762bcde33d83cadd8e7