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How to fill this surface

I've tried boundary surface, fill, and loft. How should I finish this surface? the guides ought to be obvious.
https://cad.onshape.com/documents/90204bbd7664dda0ffccb623/w/e74aafb81f7eed37d02e0cd5/e/97754ad9daa03477358a6f25?renderMode=0&uiState=68e0c1c4fccb3fab6bc891d0
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Comments
If you're asking generally how to model this part that's a pretty broad question so it's tough to answer succinctly. I see quite a few challenge areas. Is there something in particular you're trying to solve? Otherwise I recommend the surfacing lessons in the learning center and after that maybe my Surfacing Deep Dive video.
https://learn.onshape.com/catalog?query=surfacing
https://youtu.be/s-Ou2zgb5Yw
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Here's a simple trick using surfacing techniques. I would use the Fill command and creatively make a spline as a guide to help shape it.
Preselect midpoints before selecting the 3D spline tool (For some reason it doesn't let you select midpoints from within the tool but you can select beforehand)
It automatically creates a spline across the 2 points, the creative part is selecting the direction for each point.
For the top point you can select that top spline you've already drawn. For the bottom I would use the rollback bar to fit a 3 point plane I'll show that next:
make a quick 3 point plane then go back to the 3D spline tool and use the plane as the direction for the bottom point.
Use the plane as the direction for the second point, as always flip the direction using the arrows to make sure it makes sense.
Use the fill command and you see you need that rounded profile instead of a straight shot.
Use that 3D curve now as a guide in the fill tool.
You can use different weights on the 3D splines to change the curvature or add more 3D splines. But this is a basic surfacing technique that might help.
https://cad.onshape.com/documents/10e46fc630270ddfa3961692/w/6b79432064749549bad90de1/e/ac8f65e3fbf3309a1d4f9133?renderMode=0&uiState=68e7ade43fb82f380a5064e9