I received my torque arms from Ampedbikes (www.ampedbikes.com) a few days ago. Had a minor glitch in the ordering system, so I had to send payment for shipping and handling seperately - no biggie, I wasn't in a big hurry. The arms arrived, and I was very impressed! They are solid stainless (not magnetic at all!) steel that I measured at 0.1123 inch thick. The arm is actually two pieces, the torque arm itself and an insert that is shaped to fit the 10mm flats on their hubmotor (and I understand that IS the standard). The insert piece and the arm are machined (or perhaps cnc laser cut) with twenty teeth around the edge of the circular insert and the hole in the arm allowing the insert to be fitted into the arm at 18 degree increments.
That flexibility, plus the four holes in the arm should allow the user to easily fabricate a strap to rigidly secure the arm and prevent the axle from turning. A solid, quality product that I will buy again for my next ebike build.
Please note, I have no affiliation with AmpedBikes other than as a satisfied customer