Ten years ago, I hand-made one almost entirely out of aluminium stock and epoxy using widely available patterns. Lots of work, though. The mag and mag well were especially difficult. Bending metal multiple when you don't have perfectly square shapes is not that easy to get right. More recently I build a mag/well out of plastic, and it was easier to glue sheets together at angles.
You don't need a mill for making the receiver to put resin parts onto. You could cut the openings with a drill, a dremel and some files. I didn't actually have a proper drill, and I used hand files much more than I used the dremel.