So it's a little late and an idea hit me. It could be retarded or not. Could someone made an ak receiver from a cut piece of square tubing? I'm sure the stuff can be heat treated if you can find the right gauge.

Most square tubing I've seen has a much rounder radius than a standard AK receiver.

But the big issue is that the receiver is not square, the sides are not the same width as the bottom.

ETA: To clarify, the sides are not uniformly the same height, they taper along the length. You would need to find rectangular tubing of the right width and carefully mill down the sides. At which point you might as well mill yourself a receiver out of bar stock.