I'd looked at flap locking systems before, but though the bolts and flaps are simple, rugged, and easy to machine with conventional shop equipment, making the locking recesses in the receiver is a different problem. Also, I've never been able to examine a receiver from a flap-locked rifle, and so far haven't found any good pictures. The flap lock seems so nifty, but without a way to make the recesses in the receiver it's just a curiosity.
One of those magic endmills that cuts around a 90 degree corner would sure help...
Anyway, I was sketching some stuff this morning, trying to figure out if I could reach in through the barrel hole with a long end mill (no) or cut the receiver off into an AK-style trunnion and do it from the back with a keyseat cutter (no).
With only some poor pictures of an RPD, it looks like the Russians used a big side cutter to make most of the recess, then cut a hole in the top rail and broached or shapered the end of the recess square.
The British EM-2 used flap locks. Best as I can tell, the EM-2 receiver was stamped in left and right halves and welded together. It would have been easy to finish the stamped recesses before welding.
If you bolted a receiver together with side plates like a 1919 a flap lock would also be easy.
I doodled some stuff involving milled holes in the side of the receiver, where squared recess "buttons" could be pressed in and welded. That would work, but it lacks elegance.
Then I had another idea - you could just tilt the receiver or cutter at an angle and mill the recess from outside, where you would end up with a big hole with the front and back edges slanted a bit forward, at whatever angle the flaps rested at.
The front of the flap is restrained by the bolt, so it's not going to fall into the squared-out hole. The back of the flap needs to be stopped to keep it from flying out past the side of the receiver. A cover plate would work... or you could do an AK-ish trunnion and wrap it in a sheet metal receiver, which would be a fine cover plate.
I have a scratch-build project ongoing with roller locking. The plan was to go to a conventional turnbolt lock if the rollers didn't work, but the flaps would be a better fallback.