Rectangular basket.
As promised I tried out a rectangular one.
Starting with a rectangle you never get a rectangle, always a parallelogram or diamond. I should have realised that to start with!
So I drew the bottom of the basket, added sides, then flaps, see picture 1.
Pictures 2 and 3 show the results.
It's clear that the triangles along the long sides are far too big, so I drew a red line to get them the same hight as the other ones. Unfortunately that means you have to start with an odd shape, not a simple square or oblong.
What works well with a square does not work for a rectangular basket.