Actually maybe I got an idea from that hardened nut.
if you can solder hardened steel, then you can solder the nut to something that goes in a vise and then it can be filed nicely. Then you have a hardened holder that the file should skid off of once you get near level. i forgot i made one like 8 years ago for 1/4
for big bolts you can braze a washer into a nut to make a bolt that holds a smaller conical head screw and fits good in a vise
Better for filing might be to make the same mechanism but to have a big bolt that goes under the little nut so you can put the 2-56 bolt in there and tighten it from the bottom so it does not move like a clamp.
maybe
big nut -> small washer -> hardened nut. Then you put a big bolt into the big nut after you insert a tiny screw into the hardened nut to keep it absolutely stable.
But I wonder if it will hold or pop open.
Or instead of a nut you use a cap. maybe they call a full metal thread on cap a bolt protector or something, then its only one piece. Like a hydrualic cap with a hole drilled in it.
a large thread blind nut would work
drilled Brass flat top cap nut with a hardened 2-56 nut soldered on top over the hole. !!!!
or just thread the nut and glue a hardened washer on top to protect the brass then it should file flat and level. But the brass won't deburr and the hardened nut might help it after filing or sandin, and it can be real thin so long you hold it on the bottom with a bolt. I just dont wanna deal with making a hexagon cutout to secure it properly
oh i know you can probobly use a circular broach nut and glue it in to a cylinder bore, but that might pop out from the pressure from the back
needs nasa
or maybe you can spot weld the nut on. i bet that might work and leave it hard enough despite melting on the corners.