I’d start with the shape of the box itself, i.e. some beveled edges:
Then use the knife tool to outline the shape of the corner piece:
Add some loops to get a half-way clean geometry (this could be improved):
Select only the faces belonging to the corner piece, duplicate and separate:
Add a subdivision modifier to smooth out the shape and a solidify modifier to give it thickness
Add another subdivision surface or a bevel modifier to round of the edges:
suitcase_corner.blend (120.9 KB)