This is really quite easy and a quick fix. I does mean you'll be able to only use one lock ring but with adequate torque and some BLUE Loctite the motor ought to stay put.
I've always been able to put enough force on with a Hozan Lockring Wrench. I have a nice rubber mallet and give the Hozan C-205 a couple of solid taps. Of course here's where the Bafang tool with a 1/2 drive breaker bar really shines!

Early adopters were sold a "kit" which was nothing more than longer M6 screws, 20MM, and a 5mm stack of washers. I'm attaching the photos available by googling the install.

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