Hi zsouthboy - for sweeping a thing like that directly you would normally need to set up the curves so that there were 2 curves and then use those 2 as rails for the sweep. That would probably mean joining some curves and using Trim at the mid-way point to cut the one on the back into 2 pieces.

But yeah if I understand correctly you would normally want to have a solid there and then trim off a piece of the solid with a side profile curve.

This previous post may help some:
http://moi3d.com/forum/index.php?webtag=MOI&msg=1359.2

I'll see if I can cook up some examples for your specific case here though - normally for cutting a solid I guess you would build this as a kind of bigger block to start with instead of building it up with those thin walls on that upper piece to start with.

- Michael