Can anyone tell me the best way to miter/cut/chamfer the top angled edge with the sides as well as the bottom edge (with the sides) so that the part flat patterns and the corners match with a slight gap when un-flattened?
One method would be to edit the flange sketch by clicking the "Edit Flange Profile" button in the Flange property manager. You can then move the edges of the side flanges to be parallel to and offset from the top angled flange.
However, you cant have a flat pattern that will fold into the part you have drawn with the flange corners all coming together.
The flat cannot be made for what you want. As Daen stated you cannot unfold into itself. Your design would need to be made from multiple parts. Shown below is about the best you could manufacture from a single part. Possibly look at a deep draw.
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