Hi Rudy,

With two long curves, they can be extruded and the top 2 edges Blended.

Your curves have several sections, and only two long ones are desired.
I deleted everything except the bottom curves. Place a point to preserve extrude distance, or remember the height.
Trim the two extreme rounded ends in the middle with two points.
Save only the long curvy side, and rebuilt it to get rid of "corner" type points near the 2 ends. I used .001 tolerance.
Then mirror the long curve.
Extrude the two long curves.
Select the two top EDGES and BLEND. Blend can be adjusted.
Planar the bottom, and Join all 4 surfaces to a solid.

- Brian

BlendCap script is another way, with the correct single top edge.