as long as the spline is just 2D you can use sysvar PLINECONVERTMODE and set it to 1 >>>details<<< command _SPLINEDIT option convert to Polyline That creates a polyline based on arcs. _FLATTEN from my point of view is too destructive, I only use that as last option of nothing, real...
If you only have AutoCAD then you will have to go to Plan B. Create the TIN by manually defining 3dfaces. After creating the faces you can convert them to regions. At this point you will have the top surface. You can use a flat plane for the bottom surface and...