what is the best open source software for nest complex 2D shapes?.. and what about other paied software?
Materialise Magics has a nesting module that works well. It handles both 2D and 3D problems. It also allows options for bounding box vs. more complex solutions. But it is rather expensive.
For free options, look online for programs that optimize cuts from sheet products (i.e. plywood or cloth).
Many 3D printers come with software which does nesting operations. You'd just need to see if you can create a custom machine size to match the size of product you are working with.