draw the piece and rebuild the curves ( few points is better )

duplicate and align

between the 2 drawings ... place a serie of profiles ( they can be different if you need to obtain variation in final surface ) it's the case on your photo

( the result on the second pict where it's an arrow )

select all profiles and sweep 2 rails ( the 2 rails are your drawings )

to close the guy select each drawing and apply planar

select the 3 surfaces and join ... it's solid