I made a small tutorial. more hints for similar problems you will finde here..
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use GSD and multisection surface
select the vurves
replace the closing points
look at the orientation of red arrow, complete all 4 sketchs
use a guide curve for a better result
use couplings for even more better result.
Your sample didn´t do that, you can see a bad surface in the middle.
just add video will be awesome..
Here a photo of the model