Glad you didn't crash as a result, and thank you for again pointing out how important it is to have proper size and fit of torque arms.
I have hand fit torque plates of my own using a file and dremel making it just loose enough to fit with out tapping it on with a hammer, and STILL had trouble with a 12 MM axle on my NuVinci, and this was before I had a motor installed, leg power alone.
It's easy to understand once you have experienced it, but hard to fathom how much torque is really being put on such a small area on those axle flats until you have it happen first hand!
I would also recommend some form of hardening of the steel after you have made the plate, especially with a 12 MM axle. 14 MM axles are not anywhere near as prone to failure from what I have seen so far in my limited experience, however I had to harden and temper the torque plate on the smaller axle side (left side) of the NuVinci, with minimal power (leg power).
Now even though I peak at around 2500W of load, 1100 constant, I have not had any trouble since I hardened the plate.
Fortunately for me, the NuVinci has very hard and tough axle material, and it actually dug into the plate making a portion of the plate "threaded" before partially flattening a part of the threads.
Nothing wrong with any of the common torque arms sold with E-Bike kits that I have seen, just that there are limits to them, and anything with near 3x the rated wattage is going to cause most any commercial torque arm to fail save Doctor Bass's and other people's heavy duty custom built ones.
The difficult part if you don't have a proper machine shop like me, is working the steel if it's already hardened, much easier to build with it mild, and then go back and harden it later.
I have had really good luck so far with torque plates built from a cheap tow hitch receiver from a pawn shop I got for like $10, and it's probably not just mild either, it's a step somewhere in between, and after about 3 attempts, I got it to harden significantly, and 546 miles later, it's still doing fine.