Here’s some pics taken of my clone Mod1 upper of the area that counts in this discussion. You can extrapolate the rest from using these pics and sketching what you see. The thread pattern for the brake exterior is M22x1.0. The tap and proper bit isn’t hard to come by.
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If I were you I would go either of these routes, depending on what you want the finished product to look/feel like.
1: Get solid aluminum 1.5” diameter, 8-13/16” long(finished) and have the center bored for the bullet, one end bored for the muzzle brake threads and then bored for the collar diameter with a 20° taper at the end.
2: Buy the adapter I did and get a solvent trap tube to match. Have a tube for the bullet passage welded in place and welded to the endcap. This could be done with steel and not be so heavy as a solid chunk.
Note: I’m not attesting to any legalities here. I’m building a Form 1 can with the adapter shown and have zero parts otherwise so I’m good. YMMV.
Hope this helps