When I have to model something like that with complex curvature, I start by -
taking some photographs looking directly at top, front, side and other views as needed. (one looking directly at that inclined planar opening face would be included)
Try to get as perpendicular as possible, and experiment with distances and camera settings to eliminate as much parallax error as possible.
Place each image in it's own sketch and scale to get to 1:1 relative to real world size of some key feature.
Lock down the sketch(es).
I do not place any part sketch geometry on these sketches.
When creating the part sketches - use the images only for reference - not absolute truth, as there will be some parallax error.
The pictures you have now supplied would have greatly helped at the outset - but parallel/perpendicular perspective shots would really help now.
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