Hi billis, I'm not quite sure if I understand the result you want, but if you want the top to be kind of rounded a bit rather than totally flat, one other way to do that is to build a rounded "cap" surface:

Taking 2 crossing curves, select the first one as the profile:



Then run Construct / Sweep and pick the other one as the rail curve, that will generate this kind of rounded sheet:




You want to make sure that sheet is large enough to extend all the way across your bottom curves.

Then pick your original bottom curves, and run Construct/Extrude to punch them up into a solid, make sure it goes high enough to cross over your rounded sheet:



Then select the extrusion, run Construct / Boolean / Diff, and select the rounded sheet as the cutting object.

This will cut your extruded solid into 2 pieces:



Select the top piece and delete it, and you will be left with an extrusion that has a kind of rounded top instead of a flat top:



I've also attached the 3DM model here as rounded_top_cross_3dm.zip .

If that's not what you were looking for, I think I'll need some more description from you of what you are trying to do.

You can also make a lot of variations in the shape by changing how that top cutting surface is formed.

Hope this helps!

- Michael