Hi Pilou, if I understand you correctly, you will want to extract these 2 curves (select them and then use Copy + Paste):



Then go to the Front view and use the edit frame to flatten them down to be planar, like this:



Now go to the Top view and use Construct / Offset to generate an offset curve:




That offset curve will become the new outline - you will then need to use Edit/Separate on the current solids to break them into surfaces, delete the side walls, untrim the surface, retrim one of the top or bottom surfaces with the offset curve, and then use Shell to thicken that surface back to a solid again.

- Michael