Machining out a basic blank receiver is a simple thing, if you have a CNC machine and the line drawings. All you do is input and set up. Might take about 30 hours of solid machine work with a 20 bit auto change head CNC mill. Doing it by hand on a knee or smaller would take you until rapture and be just a smidge out of whack everywhere...I bet. I'd buy a receiver and get changing.

My last BAR was a NESA, not nearly s pretty as this one though.