I would suggest for doing ONE build that you use a bent metal blank. It is simply a matter of drilling some holes and cutting out the rest w/a dremel. You dont need to have a gunsmith headspace it for you, it is pretty simple if you have access to a press. IF NOT then I suppose its OK to have them do it.
The only way to "properly" bend a flat is to use some kind of a brake or at the bare minimum some parallel steel stock to clamp the flat into and provide a square, sharp edge for bending it.... If you want to do a few it would be better/faster to do bending of flats. If you want just one for yourself to make sure you are into the whole scene, just use a bent metal blank.
I THINK someone needs to make a bent metal blank that has nothing cut out but the safety hole (the big hole w/slot on right side) and the magwell opening. Then, the home builder would just need to drill the rest of the holes, set the OAL to their needs (I would make them longer than a real receiver so it takes a little trimming to make them work...80% is only a state of mind ya know LOL ) install the rails and have at it.
IS there any reason you couldnt use quality steel pop-rivets for the lower rails? Seems to me they only act as secondary guides and the ejector doesnt have much force against it. Im thinking I would like to do steel pop rivets so you can easily replace the rail if the ejector wears or needs to be moved a little for proper functioning. I dont want to use screws because they can loosen and get lost. Maybe I can just pop-rivet it to check function then plug weld them also to provide a little strength????
GOOD LUCK BRO!