Why not cut the barrel extension off an old barrel then drill and tap like the ones you can buy?Only way to make a barrel extension that I know of is if you own a broach to cut the lug recesses-- plus you need to make sure it is the proper material and harden it to the proper Rockwell hardness after you broach cut the lugs. The extension is probably the hardest part to make. I know guys that do their own blanks, but every one of them that I know buys their extensions.
Why not cut the barrel extension off an old barrel then drill and tap like the ones you can buy?