Hi JL, so the tricky part is the surface of a cello isn't just a flat plane and it isn't even curved in only one direction like extruding a 2D curve straight out will produce.

To make a curved surface like that you would need to position some cross section profile curves and then use a surface construction tool like Loft:
http://moi3d.com/3.0/docs/moi_command_reference7.htm#loft
or Sweep:
http://moi3d.com/3.0/docs/moi_command_reference7.htm#sweep
to construct a curved surface.

The other thing is that you generally will want to make the loft to be a larger sheet which then will get trimmed to your top 2D outlines, rather than trying to construct a surface directly to those outlines. See here for an example:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4059.2

- Michael